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UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-K

For the fiscal year ended March 31, 2005

OR

For the transition period from                      to                     

Commission file number: 0-14656

REPLIGEN CORPORATION

(Exact name of Registrant as specified in its charter)

41 Seyon Street Building #1 , Suite 100, Waltham, Massachusetts 02453

(Address of Principal executive offices) (Zip Code)

Registrant’s telephone number, including area code: (781) 250-0111

Securities registered pursuant to Section 12(b) of the Act: None

Securities registered pursuant to Section 12(g) of the Act:

Common Stock, $0.01 Par Value Per Share

Series A Junior Participating Preferred Stock Purchase Rights

(Title of Class)

Indicate by checkmark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days.    Yes  x    No  ¨.

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is not contained herein, and will not be contained, to the best of registrant’s knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K.  ¨

Indicate by check mark whether the registrant is an accelerated filer (as defined in Rule 12b-2 of the Act).     Yes  x    No  ¨.

The aggregate market value of the voting and non-voting common equity held by non-affiliates as of September 30, 2004, the last business day of the registrant’s most recently completed second fiscal quarter, was approximately $53,067,887.

The number of shares of outstanding of the registrant’s common stock as of June 10, 2005 was 30,094,435.

DOCUMENTS INCORPORATED BY REFERENCE

The Company intends to file a definitive proxy statement pursuant to Regulation 14A within 120 days of the end of the fiscal year ended March 31, 2005. Portions of such proxy statement are incorporated by reference in Part III of this Form 10-K.

PART I

Item 1. BUSINESS.

The following discussion of our business contains forward-looking statements that involve risks and uncertainties. When used in this report, the words “intend,” “anticipate,” “believe,” “estimate,” “plan” and “expect” and similar expressions as they relate to us are included to identify forward-looking statements. Our actual results could differ materially from those anticipated in these forward-looking statements and are a result of certain factors, including those set forth under “Certain Factors that May Affect Future Results” and elsewhere in this Annual Report on Form 10-K.

We are developing novel therapeutics for the treatment of diseases of the central nervous system. Some of our investment in these product candidates is being funded from the profits from two products which we are currently marketing. We also own intellectual property on two biological therapies which may provide future revenues to support our product development efforts in neuropsychiatric disease.

Our business strategy is to maintain full commercial rights to our product candidates through “proof of principle” clinical studies after which we may seek corporate partners for further development and marketing. We partially fund the development of our proprietary therapeutic product candidates with the profits derived from the sales of our commercial products. This will enable us to independently advance our product candidates while at the same time minimize our operating losses.

We were incorporated in May 1981, under the laws of the State of Delaware. Our principle executive offices are at 41 Seyon Street, Waltham, Massachusetts 02453 and our telephone number is (781) 250-0111.

Currently Marketed Products

We currently sell two products: Protein A products which are used in the production of monoclonal antibodies and SecreFlo^™, a synthetic form of the hormone secretin which is used as an aid in the diagnosis of certain pancreatic disorders.

Protein A Products for Antibody Manufacturing

Protein A is widely used in the purification of therapeutic monoclonal antibodies. Most therapeutic monoclonal antibodies are manufactured by the fermentation of mammalian cells that express the monoclonal antibody. The antibody product is typically produced by a process in which an impure fermentation broth containing the desired antibody product is passed over a solid support to which Protein A has been chemically attached or “immobilized”. The immobilized Protein A binds the antibody product while other impurities are washed away. The antibody is then recovered from the support in a substantially purified form.

We manufacture and market several products based on recombinant forms of Protein A. Our primary customers incorporate our Protein A products into their proprietary antibody purification systems that they sell directly to the biotechnology and pharmaceutical industry. In February 2005 we announced an amended and expanded Supply Agreement (“the Agreement”) with GE Healthcare (“GEHC”), the leading supplier of purification products to the biopharmaceutical industry and the largest consumer of Protein A. The Agreement calls for Repligen to be the primary supplier of Protein A to GEHC through 2010. We are also collaborating with GEHC to scale-up the production of a modified form of Protein A which may provide additional value to the producers of monoclonal antibodies. The majority of our product sales for the last three years have been sales of Protein A products.

Sales of therapeutic monoclonal antibodies have increased from $300 million in 1997 to approximately $9.4 billion in 2004. This growth is based on the increasing use of therapeutic antibodies, including Erbitux^® for colon cancer, Synagis^® for RSV infection and Remicade^® for Crohn’s disease and arthritis. There are more than 100 additional monoclonal antibodies in various stages of clinical testing which may lead to additional growth of the antibody market and in turn, increased demand for Protein A.

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SecreFlo^™ for Pancreatic Diagnosis

In October 1999, we licensed exclusive commercial rights to a diagnostic product based on a synthetic form of porcine (pig-derived) secretin, which we market as SecreFlo^™, from ChiRhoClin, Inc. (“ChiRhoClin”), a private company. ChiRhoClin is our sole supplier of SecreFlo^TM. SecreFlo^™ is approved by the U.S. Food and Drug Administration (“FDA”) as an aid in the diagnosis of chronic pancreatitis and gastrinoma (a form of cancer) and as an aid during endoscopic retrograde cholangiopancreatography (“ERCP”), a gastrointestinal procedure. In 2004 we terminated our agreement with ChiRhoClin for breach and filed an arbitration proceeding against ChiRhoClin for their alleged failure to meet certain obligations related to product and clinical development. In May 2005 we announced the settlement of the arbitration proceeding through an agreement by which we will continue to sell SecreFlo^™ for the next several years (For more information about recent developments regarding SecreFlo^™, please see “Patents, Licenses and Proprietary Rights—SecreFlo^™”).

Intellectual Property on Monoclonal Antibody Products

Erbitux^®

Erbitux^® is an antibody developed by ImClone Systems (“Imclone”) which was approved by the FDA in February 2004 for the treatment of certain forms of colon cancer. We believe that Erbitux^® is manufactured with a cell line created by a company whose assets were subsequently acquired by Repligen. This cell line contains certain patented genetic technologies (“DNA enhancers”) which increase the productivity of a cell line. This patent is assigned to MIT and exclusively licensed to Repligen. Imclone has announced that it manufactured approximately $1 billion of Erbitux^® as of February 2004 which will be sold over the next several years. In May 2004 Repligen and MIT filed a lawsuit against Imclone alleging that Imclone has infringed our patent rights in its production of Erbitux^®. Our patent expired in May 2004 and we have applied for a 5 year term extension for the patent or until May 2009.

CTLA4-Ig

CTLA4 is a key regulator of the activity of the immune system which “turns off” the immune system after it has successfully cleared a bacterial or viral infection by blocking the activation of T-cells, the immune cells responsible for activating an immune response. In the 1990’s our collaborators at the University of Michigan and the U.S. Navy demonstrated in animal models that a soluble form of CTLA4 (“CTLA4-Ig”) could be used to block organ transplant rejection and treat certain autoimmune diseases. Additional animal and human studies by many other groups have confirmed that CTLA4-Ig may be useful in treating diseases such as rheumatoid arthritis, multiple sclerosis, lupus, psoriasis and organ transplant rejection. CTLA4-Ig’s mechanism of action is different from any current therapy for autoimmune disease or organ transplant rejection, thus it may provide a treatment for patients who are refractory to current therapies.

In February 2004 we were issued a U.S. patent covering the use of CTLA4-Ig for the treatment of rheumatoid arthritis, multiple sclerosis and lupus. This patent is in force until 2021. In August 2004 we were issued a European patent covering the use of CTLA4-Ig for the treatment of autoimmune disease including rheumatoid arthritis as well as organ transplant rejection. This patent is in force until 2013.

In April 2005 Bristol-Myers Squibb (“Bristol”) announced that they had completed the filing of a Biological License Application with the FDA in March 2005 for the use of CTLA4-Ig for the treatment of rheumatoid arthritis and had previously stated that this application has been given “Fast Track” designation by the FDA. If the FDA approves Bristol’s CTLA4-Ig we will seek to license our patent to them.

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Development Stage Products for Neuropsychiatric Disorders

Schizophrenia

Schizophrenia is a serious, disabling and chronic mental disorder that affects 2 million people in the United States. Schizophrenia is characterized by thought disorders such as delusions or hallucinations, as well as social withdrawal, lack of initiative, and cognitive deficits. Current antipsychotic drugs are effective in reducing thought disorders in some patients but have limited effects on the social withdrawal or cognitive symptoms. The total cost for the care and treatment of patients with schizophrenia in the United States in 2000 was $40 billion.

Secretin is a hormone produced in the small intestine that regulates the function of the pancreas as part of the process of digestion. More recently, secretin and its receptor have been found in the central nervous system, suggesting a possible role as a neurotransmitter. In February 2005 we reported initial data from our double-blind, placebo-controlled Phase 2a clinical trial to evaluate secretin in patients with refractory schizophrenia. In this study patients received an intravenous dose of secretin or a placebo twice a week for two weeks. The Phase 2a study produced mixed results with a trend towards improvement in a physician’s overall assessment of the patients called Clinical Global Impression or CGI; however, no differences were observed between the groups with the Positive and Negative Syndrome Scale, a commonly used scale which assesses the core symptoms of schizophrenia. Similar results were seen in a Phase 1 study conducted by investigators at the University of North Carolina. In our Phase 2a study several patients engaged in a quantitative cognitive testing paradigm after their first dose to assess the impact of secretin on a cognitive deficit characteristic of schizophrenics, which would not be expected to be captured by the standard symptom scales. The results suggested that further investigation of the impact of secretin on this symptom is warranted. We are currently designing a follow-up study to determine if this observation is reproducible and drug-dependent.

Anxiety Disorders

Anxiety disorders including Obsessive-Compulsive Disorder, Post Traumatic Stress Syndrome and Generalized Anxiety Disorder affect more than 20 million people in the United States each year. We have demonstrated that one of the targets of secretin in the brain is the amygdala, a region which is recognized as the center of the fear and anxiety response. In a recent publication, we demonstrated in collaboration with academic researchers that secretin is active in an animal model of anxiety.

We are currently conducting a Phase 1 clinical study of secretin in an anxiety disorder called Obsessive-Compulsive Disorder in which we are increasing the dose frequency to three times a week and using a subcutaneous dose form designed to increase the total exposure of the patient from each dose by 5-fold compared to the intravenous or IV dosing used in the schizophrenia and autism studies. Our subcutaneous formulation provides a significantly longer exposure to the drug with equal or fewer side-effects which may impact the duration of potential benefits.

Bipolar disorder

Bipolar disorder, also known as manic depression, is marked by extreme changes in mood, energy and behavior in which a person can alternate between mania (highs) and depression (lows). Bipolar disorder affects more than 2 million adults in the United States. Current drug therapy for bipolar disorder includes the use of lithium or valproic acid. However side effects are frequent and troublesome, and patients do not respond fully, leading to frequent recurrences of mania and depression.

Uridine is a naturally occurring molecule essential for the synthesis of DNA, RNA and many aspects of protein, lipid and carbohydrate synthesis and metabolism. We are evaluating the potential of uridine for bipolar disorder and major depression. Preclinical studies conducted by academic researchers have demonstrated that uridine is active in an established animal model of depression. We have completed a Phase 1 trial of a prodrug of uridine in patients with bipolar disorder or major depression. This study showed that the use of uridine in these

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patients was well tolerated and provided preliminary evidence of a clinical effect in bipolar patients. We are currently conducting a pilot study to assess the bioavailability of a solid dose formulation of uridine. If successful, we may initiate a placebo-controlled, Phase 2 trial of uridine in patients with bipolar disorder.

Repligen’s Business Strategy

Our business strategy is to retain full commercial rights to our product candidates until we demonstrate “proof of principle” in controlled clinical trials, after which we may elect to complete product development or seek a development and marketing partner. In order to cost effectively advance our portfolio of product candidates, we seek to manufacture clinical trial materials through outside contract vendors and to partially fund the development of our proprietary therapeutic product candidates with the profits derived from the sales of our commercial products: Protein A and SecreFlo^™. This will enable us to independently advance our proprietary drug development programs while at the same time minimize our operating losses.

Sales and Marketing

We sell our Protein A products primarily through value-added resellers including GE Healthcare, and Applied Biosystems, Inc. as well as through distributors in certain foreign markets. We market SecreFlo^™ directly to gastroenterologists in the United States.

Customers

Customers for our Protein A products include chromatography companies, diagnostics companies, biopharmaceutical companies and laboratory researchers. During fiscal year 2005, the customers that accounted for more than 10% of our total revenue were GE Healthcare, Applied Biosystems and Cardinal Healthcare. During fiscal 2004, the customers that accounted for more than 10% of our total revenue were GE Healthcare and Cardinal Healthcare. During fiscal year 2003, we commenced selling SecreFlo^™, a diagnostic product that is marketed in the U.S., to hospital-based gastroenterologists.

Geographic Reporting

Of our fiscal 2005 revenue, 43% is attributable to U.S. customers and 56% is attributable to foreign customers, of which 77% is attributable to three customers. Of our fiscal 2004 revenue, 50% is attributable to U.S. customers and 50% is attributable to foreign customers, of which 64% is attributable to three customers. Of our fiscal 2003 revenue, 46% is attributable to U.S. customers and 54% is attributable to foreign customers, of which 66% is attributable to two customers.

Employees

As of June 7, 2005 we had 37 employees. Of those employees, 27 were engaged in research, development and manufacturing and 10 in administrative and marketing functions. Eleven of our employees hold doctorates or other advanced degrees. Each of our employees has signed a confidentiality agreement. None of our employees are covered by collective bargaining agreements.

Patents, Licenses and Proprietary Rights

Our policy is to seek patent protection for our therapeutic product candidates. We pursue patent protection in the United States and file corresponding patent applications in relevant foreign jurisdictions. We believe that patents are an important element in the protection of our competitive and proprietary position, but other elements, including trade secrets, orphan drug status and know-how, may also be important. We own or have exclusive rights to more than 15 issued U.S. patents and corresponding foreign equivalents. The terms of such patents expire at various times between 2009 and 2021. No patent material to our business expires before 2009. In

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addition, we have rights to more than 20 U.S. pending patent applications and corresponding foreign applications. The invalidation of key patents owned or licensed by us or the failure of patents to issue on pending patent applications could create increased competition, with potential adverse effects on our business prospects. For each of our license agreements where we license the rights to patents or patent applications, the license will terminate on the day that the last to expire patent covered by each such license agreement expires.

We also rely upon trade secret protection for our confidential and proprietary information. Our policy is to require each of our employees, consultants, business partners and significant scientific collaborators to execute confidentiality agreements upon the commencement of an employment, consulting or business relationship with us. These agreements generally provide that all confidential information developed or made known to the individual during the course of the individual’s relationship with us is to be kept confidential and not disclosed to third parties except in specific circumstances. In the case of employees and consultants, the agreements generally provide that all inventions conceived by the individual in the course of rendering services to Repligen shall be our exclusive property.

Secretin

We are currently prosecuting patent applications for the use of secretin for the treatment of anxiety disorders and schizophrenia in the United States and key foreign markets. In March 2003, the University of North Carolina (“UNC”) filed patent applications claiming the use of secretin for the treatment of certain other behavioral disorders, including schizophrenia. In March 2004, we exclusively licensed UNC’s rights in this area, which is unrelated to SecreFlo^™.

CTLA4-Ig

We are the exclusive licensee of all CTLA4-Ig patent rights owned by the University of Michigan (“Michigan”). In 1992, we licensed the rights to certain CTLA4 related patent applications from Michigan. In 1995, we assigned these patent rights to Genetics Institute, Inc. In 1996, Genetics Institute, Inc. returned to us the entirety of those rights which relate to CTLA4-Ig. In 1999, we executed an agreement with Genetics Institute and Michigan which ratified Michigan’s grant of an exclusive license of the CTLA4-Ig rights to us. In February 2004, a U.S. patent (the “CTLA4-Ig Use Patent”) issued, to which we own the exclusive rights through license agreements with Michigan and the U.S. Navy. The CTLA4-Ig Use Patent has claims that cover the use of CTLA4-Ig to treat rheumatoid arthritis, multiple sclerosis, and certain other autoimmune disorders and is assigned to Michigan and the U.S. Navy. The CTLA4-Ig Use Patent expires in 2021. In August 2004 we were issued a European patent covering the use of CTLA4-Ig in the treatment of autoimmune disease including rheumatoid arthritis as well as organ transplant. This patent is in force until 2013. Under the European system third parties can file oppositions to patents during the nine months following issuance of a European patent. On May 4, 2005, Bristol filed an opposition to our European patent which initiates the opposition process.

Uridine

In November 2000 and December 2000, Repligen entered into two license agreements (the “UCSD Uridine License Agreements”) with the University of California, San Diego (“UCSD”) for certain patent applications pertaining to the use of uridine and uridine derivatives for the treatment of mitochondrial disease and purine autism. On June 21, 2001, Pro-Neuron, Inc. filed a complaint (the “Pro-Neuron Complaint”) against the Regents of the University of California (the “Regents”) and Repligen in the Superior Court of California, County of San Diego seeking to void the UCSD Uridine License Agreement relating to treatment of mitochondrial disease entered into between Repligen and the UCSD. Pro-Neuron subsequently amended the complaint to include the UCSD Uridine License Agreement related to purine autism and claims for misappropriation of trade secrets.

In June 2003, Repligen agreed to restructure the UCSD License Agreements to exclude the field of acylated pyrimidines, including triacetyluridine, which we sometimes refer to as RG2133. (For more information on our intellectual property rights to uridine and related compounds for the treatment of mitochondrial disease, please see “Legal Proceedings.”)

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In April 2004, a U.S. patent was issued to Repligen and University of California, which claims methods of treating certain developmental disorders, including certain forms of autism, with uridine compositions which expires in October 2020.

Protein A

We own a U.S. patent covering recombinant Protein A, which expires in 2009, as well as significant know-how in the manufacture of high-purity Protein A. We also own a U.S. patent covering modified forms of Protein A, which was non-exclusively licensed to Amersham Biosciences (now GE Healthcare) in 1998 as part of a ten year agreement, which was amended and expanded in 2005 until 2010, covering the supply of Protein A to GE Healthcare.

In addition to its utility in antibody manufacture, Protein A may also be useful in human therapy based on its activity as a B-cell toxin. Repligen has exclusively licensed rights from the University of California, San Diego to a patent application which claims a variety of potential therapeutic uses of Protein A. Foreign equivalents of this patent application are also pending.

Research and Development

For the past three years, we have devoted substantially all of our resources on the research and development of therapeutic product candidates and our commercial products and product candidates discussed herein. We spent $5,037,000 in fiscal 2005, $6,484,000 in fiscal 2004, and $5,227,000 in fiscal 2003 on company-sponsored research and development activities.

Competition

Our Protein A and SecreFlo^™ products compete on the basis of quality, performance, cost effectiveness, and application suitability with numerous established technologies. Additional products using new technologies that may be competitive with our products may also be introduced. Many of the companies selling or developing competitive products have financial, manufacturing and distribution resources significantly greater than ours.

The field of drug development is characterized by rapid technological change. New developments are expected to continue at a rapid pace in both industry and academia. There are many companies, both public and private, including large pharmaceutical companies, chemical companies and specialized biotechnology companies, engaged in developing products competitive with products that we have under development. Many of these companies have greater capital, human resources, research and development, manufacturing and marketing experience than we do. They may succeed in developing products that are more effective or less costly than any that we may develop. These competitors may also prove to be more successful than we are in production and marketing. In addition, academic, government and industry-based research groups compete intensely with us in recruiting qualified research personnel, in submitting patent filings for protection of intellectual property rights and in establishing corporate strategic alliances. We cannot be certain that research, discoveries and commercial developments by others will not render any of our programs or potential products noncompetitive.

Manufacturing

Protein A for Antibody Manufacturing

We manufacture Protein A products from recombinant strains of bacteria. We manufacture Protein A for GE Healthcare under a ten-year supply agreement which was initiated in December 1998. In February 2005 we announced an amended and expanded Supply Agreement with GE Healthcare (“GEHC”), the leading supplier of purification products to the biopharmaceutical industry and the largest consumer of Protein A. Third parties carry out certain fermentation and certain recovery operations. The purification, certain recovery, immobilization,

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packaging and quality control testing of Protein A are conducted at our facilities. We maintain an active quality assurance effort to support the regulatory requirements of our customers. We purchase raw materials from more than one commercially established company and believe that the necessary raw materials are currently commercially available in sufficient quantities necessary to meet demand.

SecreFlo^™ (synthetic porcine secretin)

SecreFlo^™, our diagnostic secretin product, is purchased from ChiRhoClin who contracts with third parties for the synthesis of the drug substance and the drug product. This company is our sole supplier for this product. Under the terms of a settlement agreement, ChiRhoClin is obligated to deliver a certain amount of SecreFlo^™ to Repligen over the next few years. After depletion of all supplies of SecreFlo^™, including those to be delivered under the settlement agreement, Repligen will cease marketing and selling SecreFlo^™. (For more information about the settlement agreement with ChiRhoClin, please see “Legal Proceedings”).

Therapeutic Product Candidates

We currently rely, and will continue to rely, for at least the next few years, upon contract manufacturers for both the procurement of raw materials and to produce our product candidates for use in our clinical trials. Our product candidates will need to be manufactured in a facility by processes that comply with the FDA’s good manufacturing practices and other similar regulations. It may take a substantial period of time to begin manufacturing our products in compliance with such regulations. If we are unable to establish and maintain relationships with third parties for manufacturing sufficient quantities of our product candidates and their components that meet our planned time and cost parameters, the development and timing of our clinical trials may be adversely affected.

We purchase raw materials from more than one commercially established company. Our necessary raw materials are currently commercially available in quantities far in excess of the scale required to complete all of our future planned clinical trials.

Government Regulation

The development of drug candidates is subject to regulation in the United States by the FDA and abroad by foreign equivalents. Product development and approval within the FDA regulatory framework usually takes a significant number of years and involves the expenditure of substantial capital resources. Timelines for development are uncertain.

Before clinical testing in the United States of any drug candidate may begin, FDA requirements for preclinical efficacy and safety must be completed. Required toxicity testing typically involves characterization of the drug candidate in several animal species. Safety and efficacy data are submitted to the FDA as part of an Investigational New Drug Application (“IND”) and are reviewed by the FDA prior to the commencement of human clinical trials.

Clinical trials involve the administration of the drug to human volunteers or patients under the supervision of a qualified investigator, usually a physician, with an FDA-approved protocol. Human clinical trials are typically conducted in three sequential phases:

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All data obtained from a comprehensive development program are submitted in a New Drug Application (“NDA”) to the FDA and the corresponding agencies in other countries for review and approval. The NDA includes information pertaining to clinical studies and the manufacture of the new drug. Review of an NDA by the FDA can be a time-consuming process, and the FDA may request that we submit additional data or carry out additional studies.

Available Information

We maintain a website with the address www.repligen.com. We are not including the information contained on our website as a part of, or incorporating it by reference into, this annual report on Form 10-K. We make available free of charge through our website our annual reports on Form 10-K, quarterly reports on Form 10-Q and current reports on Form 8-K, and amendments to these reports, as soon as reasonably practicable after we electronically file such materials with, or furnish such materials to, the Securities and Exchange Commission.

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CERTAIN FACTORS THAT MAY AFFECT FUTURE RESULTS

Investors should carefully consider the risk factors described below before making an investment decision.

If any of the events described in the following risk factors occur, our business, financial condition or results of operations could be materially harmed. In that case the trading price of our common stock could decline, and Investors may lose all or part of their investment. Additional risks and uncertainties that we are unaware of or that we currently deem immaterial also may become important factors that affect Repligen.

This annual report on Form 10-K contains forward looking statements that involve risks and uncertainties. Our actual results could differ materially from those anticipated in these forward looking statements as a result of certain factors, including the risks faced by us described below and elsewhere in this annual report on Form 10-K.

We are dependent on others to develop, conduct clinical trials for, manufacture, market and sell our principal products.

We conduct some of our development activities, and conduct most of our commercialization activities, through collaborations. These collaborations include academic researchers as well as contracts with vendors. Our collaborations are heavily dependent on the efforts and activities of our collaborative partners. Our existing and any future collaborations may not be technically or commercially successful. For example, if any of our collaborative partners were to breach or terminate an agreement with us, reduce its funding or otherwise fail to conduct the collaboration successfully, we may need to devote additional internal resources to the program that is the subject of the collaboration, scale back or terminate the program or seek an alternative partner, any of which could lead to delays in development and/or commercialization of our products.

If our clinical trials are not successful, we will not be able to develop and commercialize any related products.

In order to obtain regulatory approvals for the commercial sale of our future products, we and our collaborative partners will be required to complete extensive clinical trials in humans to demonstrate the safety and efficacy of the products. We have limited experience in conducting clinical trials.

The submission of an IND may not result in FDA authorization to commence clinical trials. If clinical trials begin, we or our collaborative partners may not complete testing successfully within any specific time period, if at all, with respect to any of our products. Furthermore, we, our collaborative partners, or the FDA, may suspend clinical trials at any time on various grounds, including a finding that the subjects or patients are being exposed to unacceptable health risks. Clinical trials, if completed, may not show any potential product to be safe or effective. Thus, the FDA and other regulatory authorities may not approve any of our potential products for any indication.

The rate of completion of clinical trials is dependent in part upon the rate of enrollment of patients. Patient enrollment is a function of many factors, including the size of the patient population, the proximity of patients to clinical sites, the eligibility criteria for the study, and the existence of competitive clinical trials. Delays in planned patient enrollment may result in increased costs and delays in completion of clinical trials.

We may not obtain regulatory approvals; the approval process is costly and lengthy.

We must obtain regulatory approval for our ongoing development activities and before marketing or selling any of our future products. We may not receive regulatory approvals to conduct clinical trials of our products or to manufacture or market our products. In addition, regulatory agencies may not grant such approvals on a timely basis or may revoke previously granted approvals.

The process of obtaining FDA and other required regulatory approvals is lengthy and expensive. The time required for FDA and other clearances or approvals is uncertain and typically takes a number of years, depending on the complexity and novelty of the product. Our analysis of data obtained from preclinical and clinical

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activities is subject to confirmation and interpretation by regulatory authorities, which could delay, limit or prevent regulatory approval. Any delay in obtaining or failure to obtain required clearance or approvals could materially adversely affect our ability to generate revenues from the affected product. We have only limited experience in filing and prosecuting applications necessary to gain regulatory approvals.

We also are subject to numerous foreign regulatory requirements governing the design and conduct of the clinical trials and the manufacturing and marketing of our future products. The approval procedure varies among countries. The time required to obtain foreign approvals often differs from that required to obtain FDA approvals. Moreover, approval by the FDA does not ensure approval by regulatory authorities in other countries (or vice versa).

All of the foregoing regulatory risks also are applicable to development, manufacturing and marketing undertaken by our collaborative partners or other third parties.

Even if we obtain marketing approval, our products will be subject to ongoing regulatory review which will be expensive and may affect our ability to successfully commercialize our products.

Even if we or our collaborative partners receive regulatory approval of a product, such approval may be subject to limitations on the indicated uses for which the product may be marketed, which may limit the size of the market for the product or contain requirements for costly post-marketing follow-up studies. The manufacturers of our products for which we or our collaborative partners have obtained marketing approval will be subject to continued review and periodic inspections by the FDA and other regulatory authorities. The subsequent discovery of previously unknown problems with the product, clinical trial subjects, or with a manufacturer or facility may result in restrictions on the product or manufacturer, including withdrawal of the product from the market.

If we or our collaborative partners fail to comply with applicable regulatory requirements, we may be subject to fines, suspension or withdrawal of regulatory approvals, product recalls, seizure of products, operating restrictions, and criminal prosecution.

If we are unable to obtain, maintain and enforce patents for our products, we will not be able to succeed commercially.

We must obtain and maintain patent and trade secret protection for those of our products and processes for which patent protection is available in order to protect them from unauthorized use and to produce a financial return consistent with the significant time and expense required to bring our products to market. Our success will depend, in part, on our ability to:

We cannot be sure that any patent applications relating to our products that we will file in the future or that any currently pending applications will issue on a timely basis, if ever. Since patent applications in the United States filed prior to November 2000 are maintained in secrecy until patents issue and since publication of discoveries in the scientific or patent literature often lag behind actual discoveries, we cannot be certain that we were the first to make the inventions covered by each of our pending patent applications or that we were the first to file patent applications for such inventions. Even if patents are issued, the degree of protection afforded by such patents will depend upon the:

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The patent position of biotechnology and pharmaceutical firms is often highly uncertain and usually involves complex legal and scientific questions. Moreover, no consistent policy has emerged in the United States and in many other countries regarding the breadth of claims allowed in biotechnology patents. Patents which may be granted to us in certain foreign countries may be subject to opposition proceedings brought by third parties or result in suits by us, which may be costly and result in adverse consequences for us.

In some cases, litigation or other proceedings may be necessary to assert claims of infringement, to enforce patents issued to us or our licensors, to protect trade secrets, know-how or other intellectual property rights we own or to determine the scope and validity of the proprietary rights of third parties. Such litigation could result in substantial cost to us and diversion of our resources. An adverse outcome in any such litigation or proceeding could have a material adverse effect on our business, financial condition and results of operations.

If our competitors prepare and file patent applications in the United States that claim technology also claimed by us, we may be required to participate in interference proceedings declared by the U.S. Patent and Trademark Office to determine priority of invention, which would result in substantial costs to us.

We are currently and may in the future be involved in expensive patent litigation or other intellectual property proceedings which could result in liability for damages or stop our development and commercialization efforts.

There has been substantial litigation and other proceedings regarding the complex patent and other intellectual property rights in the pharmaceutical and biotechnology industries. We are a party to, and in the future may become a party to, patent litigation or other proceedings regarding intellectual property rights.

Other types of situations in which we may become involved in patent litigation or other intellectual property proceedings include:

The cost to us of any patent litigation or other proceeding, even if resolved in our favor, could be substantial. Some of our competitors may be able to sustain the cost of such litigation or proceedings more effectively than we can because of their substantially greater financial resources. If a patent litigation or other intellectual property proceeding is resolved unfavorably to us, we or our collaborative partners may be enjoined from manufacturing or selling our products and services without a license from the other party and be held liable for significant damages. We may not be able to obtain any required license on commercially acceptable terms or at all.

Uncertainties resulting from the initiation and continuation of patent litigation or other proceedings could have a material adverse effect on our ability to compete in the marketplace. Patent litigation and other proceedings may also absorb significant management time, attention and resources.

For more information about the legal proceeding in which we are involved, please see “Legal Proceedings.”

We may become involved in litigation or other proceedings with collaborative partners, which may be time consuming, costly and could result in delays in our development and commercialization efforts.

We conduct some of our development activities, and conduct most of our commercialization activities, through collaborations with collaborative partners. Therefore, any disputes with such partners that lead to

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litigation or similar proceedings may result in us incurring legal expenses, as well as facing potential legal liability. Such disputes, litigation or other proceedings are also time consuming and may cause delays in our development and commercialization efforts.

We have limited sales and marketing experience and capabilities.

We have limited sales, marketing and distribution experience and capabilities. We may, in some instances, rely significantly on sales, marketing and distribution arrangements with our collaborative partners and other third parties. In these instances, our future revenues will be materially dependent upon the success of the efforts of these third parties.

If in the future we determine to perform sales, marketing and distribution functions ourselves, we would face a number of additional risks, including:

We have limited manufacturing capabilities and will be dependent on third party manufacturers.

We have limited manufacturing experience and no commercial or pilot scale manufacturing facilities for the production of pharmaceuticals. In order to continue to develop pharmaceutical products, apply for regulatory approvals and, ultimately, commercialize any products, we may need to develop, contract for, or otherwise arrange for the necessary manufacturing capabilities.

We currently rely upon third parties to produce material for preclinical and clinical testing purposes and expect to continue to do so in the future. We also expect to rely upon third parties, including our collaborative partners, to produce materials required for the commercial production of certain of our products if we succeed in obtaining necessary regulatory approvals. We believe that there is no proprietary aspect to the manufacture of our products. However, there are only a limited number of manufacturers that operate under the FDA’s regulations for good manufacturing practices which are capable of and/or approved to manufacture our products. Timing for the initiation of new manufacturers is uncertain, and, if we are unable to arrange for third party manufacturing of our products on a timely basis, or to do so on commercially reasonable terms, we may not be able to complete development of our products or market them.

We rely on a single supplier (ChiRhoClin) for our SecreFlo^™ product. Under the terms of our settlement agreement, ChiRhoClin is obligated to deliver a certain amount of SecreFlo^™ to Repligen over the next few years. After depletion of all supplies of SecreFlo^™, including those to be delivered under the settlement agreement, Repligen will cease marketing and selling SecreFlo^™. In the event that we are unable to acquire additional products, our revenues may be negatively impacted. (For more information about the settlement agreement regarding SecreFlo^™, please see “Legal Proceedings.”) We currently rely upon third parties for fermentation relating to our Protein A products.

We believe that there is no proprietary aspect to the manufacture of our commercial products. However, timing for the initiation of new manufacturers is uncertain, and, if we are unable to arrange for third party manufacturing of our products on a timely basis, or to do so on commercially reasonable terms, we may not be able to complete development of our products or market them. To the extent that we enter into manufacturing arrangements with third parties, we are dependent upon these third parties to perform their obligations in a timely manner. If such third party suppliers fail to perform their obligations, we may be adversely affected in a number of ways, including:

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The manufacture of products by us and our collaborative partners and suppliers is subject to regulation by the FDA and comparable agencies in foreign countries. Delay in complying or failure to comply with such manufacturing requirements could materially adversely affect the marketing of our products.

If we are unable to continue to hire and retain skilled personnel, then we will have trouble developing and marketing our products.

Our success depends largely upon the continued service of our management and scientific staff and our ability to attract, retain and motivate highly skilled technical, scientific, management, regulatory compliance and marketing personnel. Potential employees with an expertise in the field of molecular biology, biochemistry, regulatory affairs and/or clinical development of new drug and biopharmaceutical manufacturing are not generally available in the market and are difficult to attract and retain. We also face significant competition for such personnel from other companies, research and academic institutions, government and other organizations who have superior funding and resources to be able to attract such personnel. The loss of key personnel or our inability to hire and retain personnel who have technical, scientific or regulatory compliance backgrounds could materially adversely affect our product development efforts and our business.

The market may not be receptive to our products upon their introduction.

The commercial success of our products that are approved for marketing will depend upon their acceptance by the medical community and third party payors as being clinically useful, cost effective and safe. All of the products that we are developing are based upon new technologies or therapeutic approaches. As a result, it is hard to predict market acceptance of our products.

Other factors that we believe will materially affect market acceptance of our products and services include:

We compete with pharmaceutical and biotechnology companies who are capable of developing new approaches that could make our products and technology obsolete.

The market for therapeutic and commercial products is intensely competitive, rapidly evolving and subject to rapid technological change. Pharmaceutical and biotechnology companies may have substantially greater financial, manufacturing, marketing, and research and development resources than we have. New approaches by these competitors may make our products and technologies obsolete or noncompetitive.

We have incurred substantial losses, we expect to continue to incur losses and we will not be successful until we reverse this trend.

We have incurred losses in each year since our founding in 1981. We expect to continue to incur operating losses for the foreseeable future.

While we generate revenue from product sales, this revenue is not sufficient to cover the costs of our clinical trials and drug development programs. We plan to continue to invest in key research and development activities. As a result, we will need to generate significant revenues in order to achieve profitability. We cannot be certain whether or when this will occur because of the significant uncertainties that affect our business.

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If we do not obtain additional capital for our drug development programs, we will be unable to develop or discover new drugs.

We need additional long-term financing to develop our drug development programs through the clinical trial process as required by the FDA and to develop our commercial products business. We also need additional long-term financing to support future operations and capital expenditures, including capital for additional personnel and facilities. If we spend more money than currently expected for our drug development programs and our commercial products business, we will need to raise additional capital by selling debt or equity securities, by entering into strategic relationships or through other arrangements. We may be unable to raise any additional amounts on reasonable terms or when they are needed due to the volatile nature of the biotechnology marketplace. If we are unable to raise this additional capital, we may have to delay or postpone critical clinical studies or abandon other development programs.

Our stock price could be volatile, which could cause you to lose part or all of your investment.

The market price of our common stock, like that of the common stock of many other development stage biotechnology companies, is highly volatile. In addition, the stock market has experienced extreme price and volume fluctuations. This volatility has significantly affected the market prices of securities of many biotechnology and pharmaceutical companies for reasons frequently unrelated to or disproportionate to the operating performance of the specific companies. These broad market fluctuations may adversely affect the market price of our common stock.

Anti-takeover provisions may deter a third party from acquiring us, limiting our stockholders’ ability to profit from such a transaction.

Our Board of Directors has the authority to issue up to 5,000,000 shares of preferred stock, of which 40,000 have been reserved for issuance in connection with our stockholder rights plan, and to determine the price, rights, preferences and privileges of those shares without any further vote or action by our stockholders. We also adopted a “poison pill” stockholder rights plan that will dilute the stock ownership of acquirers of our common stock upon the occurrence of certain events. This stockholder rights plan could have the effect of making it more difficult for a third party to acquire a majority of our outstanding voting stock.

We are subject to the anti-takeover provisions of Section 203 of the Delaware General Corporation Law, which prohibits us from engaging in a “business combination” with an “interested stockholder” for a period of three years after the date of the transaction in which the person becomes an interested stockholder, unless the business combination is approved in a prescribed manner. This provision could have the effect of delaying or preventing a change of control of the Company. Section 203 and the stockholder rights plan may have the effect of deterring hostile takeovers or delaying or preventing changes in our management, including transactions in which stockholders might otherwise receive a premium for their shares over then current market prices.

Changes in the securities laws and regulations have increased, and are likely to continue to increase our costs.

The Sarbanes-Oxley Act of 2002, which became law in July 2002, has required changes in some of our corporate governance, securities disclosure and compliance practices. In response to the requirements of that Act, the SEC and the Nasdaq have promulgated new rules and listing standards covering a variety of subjects. Compliance with these new rules and listing standards have increased our legal costs and financial and accounting costs, and we expect these increased costs to continue. Likewise, these developments may make it more difficult for us to attract and retain qualified members of our board of directors, particularly independent directors.

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Item 2. PROPERTIES.

We lease approximately 25,000 square feet of space in Waltham, Massachusetts, of which approximately 10,000 square feet is manufacturing and laboratory space. The remaining space is used as office space. Our lease expires in January 2013, with options to extend for two five-year periods. During fiscal 2005, we incurred aggregate rental costs for our facility, excluding maintenance, taxes and utilities, of approximately $394,000. Our space is adequate for our current use and for the foreseeable future.

Item 3. LEGAL PROCEEDINGS.

ImClone Systems, Inc.

In May 2004 Repligen Corporation and The Massachusetts Institute of Technology (“MIT”) filed an action for patent infringement in the United States District Court for the District of Massachusetts against ImClone Systems, Inc. (“Imclone”) for infringement of a U.S. Patent No. 4,663,381 (“the Erbitux^® Patent”) based on Imclone’s manufacture and sale of the cancer drug Erbitux^®. The technology covered by the Erbitux^® Patent, which was developed and patented by MIT, covers certain genetic elements, DNA enhancers, that increase protein production in a mammalian cell. Repligen is the exclusive licensee of MIT for the Erbitux Patent. Repligen and MIT believe that Damon Biotech, a predecessor of Repligen, developed the cell line which is used to manufacture Erbitux^® in 1990 for the National Cancer Institute and used the technology which is the basis of the Erbitux Patent. Repligen seeks relief, including compensation in the form of royalties for the material Imclone sold prior to the expiration of the Erbitux Patent. Repligen and MIT have also filed an application for patent term extension for the Erbitux Patent, which if granted will extend the term of the patent to May 2009.

ChiRhoClin, Inc.

In February 2004, Repligen terminated the September 1999 Licensing Agreement with ChiRhoClin, its supplier of SecreFlo^™, based on ChiRhoClin’s alleged failure to meet its obligations under the Licensing Agreement.

On April 9, 2004, Repligen filed an arbitration demand against ChiRhoClin with the American Arbitration Association in New York seeking to recover payments made to ChiRhoClin and additional damages. In this arbitration demand, Repligen alleged that ChiRhoClin breached several of its obligations under the September 1999 Licensing Agreement including failure to use best efforts to obtain various FDA approvals and to manufacture and supply SecreFlo^™ in a timely manner. In June 2004, ChiRhoClin filed a counterclaim alleging that Repligen had wrongfully terminated the Licensing Agreement.

On May 9, 2005, Repligen entered into a Settlement Agreement (the “Agreement”) with ChiRhoClin, Inc., in full settlement of their arbitration proceedings described above. Under the terms of the Agreement, Repligen received a payment of $750,000 and will be entitled to continue to market SecreFlo^™ for the next several years under a royalty structure more favorable to Repligen than under the Licensing Agreement. ChiRhoClin is obligated to deliver a certain amount of SecreFlo^™ to Repligen over the next few years. After depletion of all supplies of SecreFlo^™, including those to be delivered under the Agreement, Repligen will cease marketing and selling SecreFlo^™. ChiRhoClin will pay Repligen a per unit royalty on all sales by ChiRhoClin of its secretin products subject to certain time and/or volume limits. Repligen is not required to pay approximately $1,170,000 of unremitted royalties to ChiRhoClin related to sales from February 2004 to March 2005. This amount which was accrued at March 31, 2005 will be recorded as other income in the quarter ended June 30, 2005. Repligen has received security for ChiRhoClin’s performance under the Agreement.

Pro-Neuron, Inc.

On June 21, 2001 Repligen was named as a codefendant with the Regents of the University of California (the “Regents”) in an action filed by Pro-Neuron, Inc. (“Pro-Neuron”) (now known as Wellstat Therapeutics

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Corporation) in the Superior Court of California, County of San Diego alleging claims of breach of contract and seeking to void the License Agreement between Repligen and The University of California. (“UCSD License Agreements”). On November 16, 2004, the Regents, Pro-Neuron and Repligen entered into settlement agreements (the “Settlement”) under which Repligen and the Regents amended the UCSD License Agreements to exclude the field of acylated pyrimidines, including triacetyluridine (“TAU”). Pursuant to the Settlement, Repligen has agreed not to initiate any additional clinical studies of acylated pyrimidines, including TAU. Repligen agreed to assign to Pro-Neuron inventions, if any, from its previously completed Phase I study in bipolar disorder/major depression, involving the use of acylated pyrimidines, but Repligen will retain the rights to any inventions for all other chemical entities. Following the execution of the Definitive Agreements, Pro-Neuron and the Regents paid Repligen $750,000.

From time to time, the Company may be subject to other legal proceedings and claims in the ordinary course of business. Repligen is not currently aware of any such proceedings or claims that it believes will have, individually or in the aggregate, a material adverse effect on the business, financial condition or results of operations.

Item 4. SUBMISSION OF MATTERS TO A VOTE OF SECURITY HOLDERS.

No matters were submitted to a vote of the security holders of the Company through the solicitation of proxies or otherwise, during the last quarter of the fiscal year ended March 31, 2005.

PART II

Market Information

Our common stock is traded over-the-counter on the Nasdaq National Market under the symbol “RGEN.” The following table sets forth for the periods indicated the high and low bid information for the common stock as reported by Nasdaq. These quotations reflect inter-dealer prices, without retail markup, markdown or commission and may not necessarily reflect actual transactions.

Stockholders and Dividends

As of June 10, 2005 there were approximately 843 stockholders of record of our common stock. We have not paid any dividends since our inception and do not intend to pay any dividends on our common stock in the foreseeable future. We anticipate that we will retain all earnings, if any, to support our operations and our proprietary drug development programs. Any future determination as to the payment of dividends will be at the sole discretion of our board of directors and will depend on our financial condition, results of operations, capital requirements and other factors our board of directors deems relevant.

Equity Compensation Plan Information

See Part III, Item 12 for information regarding securities authorized for issuance under our equity compensation plans.

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Item 6. SELECTED FINANCIAL DATA

The following selected financial data are derived from the audited financial statements of Repligen. The selected financial data set forth below should be read in conjunction with our financial statements and the related notes thereto and “Management’s Discussion and Analysis of Financial Condition and Results of Operations” included elsewhere in this report and our Annual Report on Form 10-K for the years ended March 31, 2004, 2003, 2002 and 2001.

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This annual report on Form 10-K contains forward-looking statements which are made pursuant to the safe harbor provisions of Section 27A of the Securities Act of 1933, and Section 21E of the Securities Exchange Act of 1934. The forward-looking statements in this annual report on Form 10-K do not constitute guarantees of future performance. Investors are cautioned that statements in this Annual Report on Form 10-K that are not strictly historical statements, including, without limitation, statements regarding current or future financial performance, potential impairment of future earnings, management’s strategy, plans and objectives for future operations and product candidate acquisition, clinical trials and results, litigation strategy, product research and development, research and development expenditures, intellectual property, development and manufacturing plans, availability of materials and product and adequacy of capital resources and financing plans constitute forward-looking statements. Such forward-looking statements are subject to a number of risks and uncertainties that could cause actual results to differ materially from those anticipated, including, without limitation, the risks identified under the caption “Certain Factors That May Affect Future Results” and other risks detailed in this Annual Report on Form 10-K and our other filings with the Securities and Exchange Commission. We assume no obligation to update any forward-looking information contained in this Annual Report on Form 10-K.

Overview

We are a biopharmaceutical company focused on the development of novel therapeutics for diseases that affect the central nervous system. A number of drug development programs are currently being conducted to evaluate our naturally occurring drug candidates in diseases such as schizophrenia, obsessive-compulsive disorder, bipolar disorder and neurodegeneration. In addition, we sell two commercial products, Protein A for monoclonal antibody purification and SecreFlo^TM for assessment of pancreatic disorders. In fiscal 2005, we experienced significant growth in sales and profits from our commercial products business. Our business strategy is to deploy the profits from our current commercial products and any revenue that we may receive from our patents to enable us to invest in the development of our product candidates in the treatment area of neuropsychiatric diseases while at the same time minimize our operating losses.

CRITICAL ACCOUNTING POLICIES AND ESTIMATES

While our significant accounting polices are more fully described in notes to our financial statements, we have identified the policies and estimates below as critical to our business operations and the understanding of our results of operations. The impact and any associated risks related to these policies on our business operations is discussed throughout “Management’s Discussion and Analysis of Financial Condition” and “Results of Operations” where such policies affect our reported and expected financial results.

Revenue Recognition

We apply Staff Accounting Bulletin No. 104, “Revenue Recognition” (“SAB No. 104”) to our revenue arrangements. We generate product revenues from the sale of our Protein A products to customers in the pharmaceutical and process chromatography industries and from the sale of SecreFlo^™ to hospital-based gastroenterologists. In accordance with SAB No. 104, we recognize revenue related to product sales upon delivery of the product to the customer as long as there is persuasive evidence of an arrangement, the sales price is fixed or determinable and collection of the related receivable is reasonably assured.

Additionally, during fiscal 2004 and 2003 we generated non-product revenues from sponsored research and development projects under a Small Business Innovation Research (“SBIR”) Phase I grant. Research revenue is recognized as earned under cost plus fixed-fee contracts, or on a straight-line basis over the term of contract, which approximates when work is performed and costs are incurred. Research and development expenses in the accompanying statements of operations include funded and unfunded expenses.

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Impairment Analysis of Long-lived Assets

We review our long-term assets for impairment at each reporting period in accordance with SFAS No. 144, “Accounting for the Impairment or Disposal of Long-Lived Assets”.

During fiscal 2003, under the terms of our September 1999 Licensing Agreement with ChiRhoClin, Inc. we made a milestone payment to ChiRhoClin that consisted of $1,250,000 in cash and 696,223 shares of our common stock. We recorded the fair value of the shares issued, $2,576,025, and the cash paid of $1,250,000, as a long-term intangible asset. Beginning in April 2002, we began to amortize this intangible asset to cost of revenue over the remaining term of the license, approximately seven years. We amortized $510,130 and $510,132 during the years ended March 31, 2004 and 2003.

We review our long-term assets for impairment at each reporting period in accordance with SFAS No. 144, “Accounting for the Impairment or Disposal of Long-Lived Assets”. At March 31, 2004, as a result of a dispute with ChiRhoClin, we recorded an impairment charge of $2,413,244 in our results of operations for the year ended March 31, 2004. During the year ended March 31, 2005, we amortized the remaining balance of this long-term intangible asset of $392,520 to cost of goods sold. This long-term intangible asset was fully amortized at March 31, 2005.

Inventory

We value inventory at cost or, if lower, fair market value. We determine cost using the first-in, first-out method. We regularly review our inventories and record a provision for excess and obsolete inventory based on certain factors that may impact the realizable value of our inventory. Factors we consider include expected sales volume, production capacity and expiration dates. We write down inventory that has become obsolete, inventory that has a cost basis in excess of its expected net realizable value and inventory in excess of expected requirements to cost of goods sold.

Accrued Expenses

We prepare our financial statements in accordance with accounting principles generally accepted in the United States. These principles require that we estimate accrued expenses. This process involves identifying services, which have been performed on our behalf, and estimating the level of service performed and the associated cost incurred for such service as of each balance sheet date in our financial statements. Examples of estimated expenses for which we accrue expenses include fees paid to our contract manufacturers in conjunction with the production of clinical materials and service fees paid to organizations for their performance in conducting our clinical trials. In the event that we do not identify certain costs which have begun to be incurred or we under or over-estimate the level of services performed or the costs of such services, our reported expenses for that period may be too low or too high. The date on which certain services commence, the level of services performed on or before a given date and the cost of such services are often judgmental. We make these judgments based upon the facts and circumstances known to us in accordance with generally accepted accounting principles.

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RESULTS OF OPERATIONS

The following discussion of the financial condition and results of operations should be read in conjunction with the accompanying financial statements and the related footnotes thereto.

Revenues

Total revenue for fiscal 2005, 2004 and 2003 were $9,360,000, $6,914,000 and $7,772,000. Revenues for the years ending March 31, 2005, 2004 and 2003 were primarily comprised of sales of our commercial products, Protein A and SecreFlo^TM. During the fiscal year ended March 31, 2005, 2004 and 2003 sales of our commercial products were:

Our products based on recombinant Protein A are substantially sold to customers who incorporate our manufactured products into their proprietary antibody purification systems to be sold directly to the pharmaceutical industry. Monoclonal antibodies are a well-established class of drug with applications in rheumatoid arthritis, asthma, Crohn’s disease and a variety of cancers. Sales of Protein A are therefore impacted by the timing of large-scale production orders and on the regulatory approvals for such antibodies, which may result in significant quarterly fluctuations.

During fiscal 2005, sales increased by $2,517,000 as a result of a steady rise in the demand for our Protein A products. During fiscal 2004, manufacturing problems experienced by one of our significant customers negatively impacted our sales of Protein A which decreased by $1,762,000. During the fourth quarter of fiscal 2005, a supply agreement with one of our major customers was amended to expand the scope of manufacturing and extend the term of the agreement through 2010. We therefore anticipate that sales of Protein A will continue to be strong during the next year, but will continue to be subject to quarterly fluctuations due to timing of large-scale production orders.

We began selling SecreFlo^™, our diagnostic secretin product that is marketed to gastroenterologists, during the second half of fiscal 2003. Sales of SecreFlo^™ increased $322,000 in fiscal 2005 from $1,867,000 in fiscal 2004 during which a delay in the delivery of a new lot of SecreFlo^™ from the manufacturer negatively impacted sales. The recent settlement with our sole supplier of SecreFlo^™ provides for continued supply during the next year and we anticipate sales to continue at current levels in fiscal 2006.

During fiscal 2003, we commenced work related to a SBIR Phase I grant for $100,000 that we completed in fiscal 2004. This grant funding was recorded as research revenue.

Cost of revenue

For the years ended March 31, 2005, 2004 and 2003, cost of revenue was $3,888,000, $3,248,000 and $3,480,000, respectively.

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This increase in costs of revenue of $640,000 in fiscal 2005 is directly attributable to higher product sales and a charge of approximately $200,000 for excess and expired inventory. During fiscal 2004, reduced sales from fiscal 2003 resulted in lower cost of revenue of $232,000 and lower gross profit of $626,000. Gross profitability is impacted by change in product mix and is positively impacted in periods when product sales are higher because there is increased dilution of fixed costs.

Operating expenses

Total operating expenses for fiscal 2005, 2004 and 2003 were approximately $9,634,000, $13,607,000 and $9,386,000, respectively.

Research and development expenses for fiscal 2005, 2004 and 2003 were approximately $5,037,000, $6,484,000 and $5,227,000 respectively. Research and development costs primarily include costs of internal personnel, external research collaborations and the costs associated with the manufacturing and testing of clinical materials. We currently have ongoing research and development programs that support our product candidates of secretin and uridine. In addition, we are involved with a number of early stage programs that may or may not be further developed. Due to the small size of the company and the fact that these various programs share personnel and fixed costs such as facility costs, depreciation, and supplies, we do not account for our expenses by program.

Each of our research and development programs is subject to risks and uncertainties, including the requirement to seek regulatory approvals that are outside of our control. For example, our clinical trials may be subject to delays based on our inability to enroll patients at the rate that we expect to meet the schedule for our planned clinical trials. Moreover, the product candidates identified in these research programs, particularly in our early stage programs must overcome significant technological, manufacturing and marketing challenges before they can be successfully commercialized. For example, results from our preclinical animal models may not be replicated in our clinical trials with humans. As a result of these risks and uncertainties, we are unable to predict with any certainty the period in which material net cash inflows from such projects could be expected to commence or the completion date of these programs.

These risks and uncertainties also prevent us from estimating with any certainty the specific timing and future costs of our research and development programs, although historical trends within the industry suggest that expenses tend to increase in later stages of development. Collaborations with commercial vendors and academic researchers accounted for 37%, 43% and 43% of our research and development expenses in the fiscal year ending March 31, 2005, 2004 and 2003, respectively. The outsourcing of such services provides us flexibility to discontinue or increase spending depending on the success of our research and development programs.

During fiscal 2005, research and development expenses decreased by $1,447,000. This decrease is largely attributable to the discontinuation of costs associated with our Phase III trial in autism that took place during fiscal 2004 and 2003, offset by expenses associated with our earlier stage programs that were executed during fiscal year 2005. During fiscal 2005, costs of clinical trials decreased by $736,000, clinical materials decreased by $124,000, external collaborations costs decreased by $90,000 and staffing costs decreased by $239,000. During fiscal 2004 research and development expenses increased by approximately $1,257,000. This increase was largely attributable to increased manufacturing costs of clinical materials of $331,000, an increase in clinical trials of $250,000, personnel and related costs increase of $347,000 and licensing costs of $242,000.

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Future research and development expenses are dependent on a number of variables, including the cost and design of clinical trials and external costs such as manufacturing of clinical materials. We expect our research and development expenses in fiscal 2006 to remain consistent with fiscal 2005 levels unless we acquire an additional product candidate, in which case, research and development expenses may increase significantly.

Selling, general and administrative expenses (SG&A) include the associated costs with selling our commercial products and costs required to support our research and development efforts including legal, accounting, patent, shareholder services and other administrative functions. In addition, SG&A expenses have historically included costs associated with various litigation matters.

During fiscal 2005, SG&A costs decreased by approximately $113,000, a result of decreased costs in shareholder services of $212,000 and marketing expenses of $174,000 offset by increased legal costs of $292,000 and external costs relating to compliance with the Sarbanes-Oxley Act of 2002 of $151,000. During fiscal 2004 SG&A expenses increased by approximately $551,000. Costs for professional and administrative fees, including patent, legal and insurance premiums, increased by $296,000 during fiscal 2004. In addition, marketing and distribution expenses increased by $288,000 during fiscal 2004 as a result of a full year of sales of SecreFlo^™ and the reimbursement of premarketing and launch expenses during fiscal 2003.

During fiscal 2004, we recognized a non-cash charge of $2,413,000 associated with the termination of the SecreFlo^™ license agreement. (See “Legal Proceedings” and Note 11 of Notes to Financial Statements.)

Investment income

Investment income includes income earned on invested cash balances. Investment income for fiscal 2005, 2004 and 2003 was approximately $428,000, $390,000 and $557,000, respectively. The increase of $38,000 or 10% in fiscal 2005 was attributable to higher interest rates earned as compared to fiscal 2004. During fiscal 2004, investment income decreased $167,000 or 30%, which is attributable to lower interest rates earned as compared to fiscal 2003. We expect interest income to vary based on changes in the amount of funds invested and fluctuation of interest rates.

Other income

Other income for the year ended March 31, 2005 consists of $750,000 in proceeds from a legal settlement from Pro-Neuron received in November 2004. The settlement is more fully described in Part I, Item 3 “Legal Proceedings”.

Liquidity and Capital Resources

We have financed our operations primarily through sales of equity securities and revenues derived from product sales and government grants. Our revenue for the foreseeable future will be limited to our product revenue related to Protein A and SecreFlo^™. Given the uncertainties related to pharmaceutical product development, we are currently unable to reliably estimate when, if ever, our therapeutic product candidates or our patents will generate revenue and cash flows.

At March 31, 2005 we had cash and marketable securities of $24,041,000 compared to $24,863,000, at March 31, 2004.

Cash Flows

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Operating Activities

In fiscal 2005, our cash used in operations decreased from 2004 levels as a result of our reduced net loss before non-cash charges such as depreciation, amortization and stock compensation charges. This reduced loss benefited from the significant increase in our product sales, the receipt of $750,000 from a legal settlement and reduced research and development expenses. Increased product sales in fiscal 2005 also reduced our inventory levels. Accounts receivable decreased from 2004 levels due to increased cash collections. In addition, as a result of a dispute with our supplier of SecreFlo^™ we recorded a royalty obligation but did not pay, therefore, increasing accrued expenses.

In May 2005, we announced a settlement with our sole supplier of SecreFlo^™. As a result of this settlement we will receive a payment of $750,000 in the first quarter of fiscal 2006 and will continue to market SecreFlo^™ for the next several years under a favorable royalty structure. In addition as a result of this settlement we will be relieved of our disputed obligation to pay approximately $1,170,000 in royalties related to prior sales. This will be recorded as other income in the first quarter of fiscal 2006.

In fiscal 2004, our cash used in operations increased from fiscal 2003 levels primarily due to our increased operating expenses in 2004 offset by the milestone payment of $1,250,000 paid to our supplier of SecreFlo^™ in fiscal 2003.

Financing Activities

In fiscal 2005, our purchases of property and equipment were $86,000 of which $34,000 was financed through capital leases. Purchases and redemptions of marketable securities account for the remainder of the fluctuation during fiscal 2005 and fiscal 2004. We generally place our marketable security investments in high quality credit instruments as specified in our investment policy guidelines result in timing fluctuations of purchases and redemptions.

In fiscal 2004, purchases of property, plant and equipment were $308,000. In fiscal 2003, purchases of property, plant and equipment of $1,084,000 were largely attributable to the leasehold improvements associated with our relocation to our current facility.

Investing Activities

In fiscal 2005, exercises of stock options provided cash proceeds of $31,000, a decrease from fiscal year 2004’s proceeds of $155,000. During fiscal 2004, we issued and sold 2,500,000 shares of our common stock to the Riverview Group, LLC for an aggregate consideration of $12,500,000. We received net proceeds of approximately $11.8 million after deducting the expenses of the transaction. No investing activities occurred in fiscal year 2003.

Off-Balance Sheet Arrangements

We do not have any special purpose entities or off-balance sheet financing arrangements.

Contractual Obligations

As of March 31, 2005, we had the following fixed obligations and commitments:

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Capital Requirements

Our future capital requirements will depend on many factors, including the following:

Absent an acquisition of a product candidate, we believe our current cash balances are adequate to meet our cash needs for at least the next twenty-four months. We expect to incur a similar level of expenses in fiscal 2006 as those incurred in fiscal 2005. Our future capital requirements include, but are not limited to, continued investment in our research and development programs, capital expenditures primarily associated with purchases of equipment and continued investment in our intellectual property portfolio.

We plan to continue to invest in key research and development activities. After the discontinuation of our Phase III trial in autism, we began a review of technology and product candidates that would complement our existing portfolio of development programs. We continue to seek to acquire such potential assets that may offer us the best opportunity to create value for our shareholders. In order to acquire such assets, we may need to seek additional financing to fund these investments. This may require the issuance or sale of additional equity or debt securities. The sale of additional equity may result in additional dilution to our stockholders. Should we need to secure additional financing to acquire a product, fund future investment in research and development, or meet our future liquidity requirements, we may not be able to secure such financing, or obtain such financing on favorable terms because of the volatile nature of the biotechnology marketplace.

Net Operating Loss Carryforwards

At March 31, 2005, we had net operating loss carryforwards of approximately $110,300,000 and research and development credit carryforwards of approximately $6,140,000 to reduce future federal income taxes, if any. The net operating loss and tax credit carryforwards will expire at various dates, beginning in fiscal 2006, if not used. Net operating loss carryforwards and available tax credits are subject to review and possible adjustment by the Internal Revenue Service and may be limited in the event of certain changes in the ownership interest of significant stockholders.

Effects of Inflation

Our assets are primarily monetary, consisting of cash, cash equivalents and short-term investments. Because of their liquidity, these assets are not directly affected by inflation. Since we intend to retain and continue to use our equipment, furniture and fixtures and leasehold improvements, we believe that the incremental inflation related to replacement costs of such items will not materially affect our operations. However, the rate of inflation affects our expenses, such as those for employee compensation and contract services, which could increase our level of expenses and the rate at which we use our resources.

Recent Accounting Pronouncements

In December 2004, the FASB issued SFAS No. 123R, Share-Based Payment—An Amendment of FASB Statements No. 123 and 95 (“SFAS No. 123R”), which requires all companies to measure compensation cost for

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all share-based payments, including employee stock options, at fair value, effective for public companies for annual periods beginning after June 15, 2005. Generally, the approach in SFAS No. 123R is similar to the approach described in SFAS No. 123. However, SFAS No. 123R requires all share-based payments to employees, including grants of employee stock options, to be recognized in the income statement based on their fair values. Pro forma disclosure is no longer an alternative. The adoption of SFAS No. 123R may have a significant impact on our results of operations, although it will have no impact on our overall financial position. We are evaluating SFAS No. 123R and have not yet determined the amount of stock option expense which will be incurred in future periods.

In November 2004, the FASB issued Statement of Financial Accounting Standards No. 151 (“SFAS No. 151”), Inventory Costs, an amendment of APB No. 43, Chapter 4. The amendments made by SFAS No. 151 will improve financial reporting by requiring that abnormal amounts of idle facility expense, freight, handling costs, and wasted materials (spoilage) be recognized as current-period charges and by requiring the allocation of fixed production overheads to inventory based on the normal capacity of production facilities. SFAS No. 151 is effective for inventory costs incurred during fiscal years beginning after June 15, 2005. Earlier application is permitted for inventory costs incurred during fiscal years beginning after November 24, 2004. We are currently evaluating the impact that adoption of SFAS No. 151 will have on our financial position and results of operations.

Item 7A. QUANTITATIVE AND QUALITATIVE DISCLOSURES ABOUT MARKET RISK

Interest Rate Risk

We have investments in commercial paper, U.S. Government and agency securities as well as corporate bonds and other debt securities. As a result, we are exposed to potential loss from market risks that may occur as a result of changes in interest rates, changes in credit quality of the issuer or otherwise.

We generally place our marketable security investments in high quality credit instruments, as specified in our investment policy guidelines. A hypothetical 100 basis point increase in interest rates would result in an approximate $112,000 decrease in the fair value of our investments as of March 31, 2005. However, the conservative nature of our investments mitigates our interest rate exposure, and our investment policy limits the amount of our credit exposure to any one issue, issuer, (with the exception of U.S. treasury obligations) and type of instrument. We do not expect any material loss from our marketable security investments due to interest rate fluctuations and therefore believe that our potential interest rate exposure is limited. We intend to hold these investments to maturity, in accordance with our business plans.

As of March 31, 2005, we did not have any debt arrangements that were not reflected in our balance sheet.

Item 8. FINANCIAL STATEMENTS AND SUPPLEMENTAL DATA

Financial statements and supplementary data required by Item 8 are set forth at the pages indicated in Item 15(a) below.

None.

Item 9A. CONTROLS AND PROCEDURES.

(a) Disclosure Controls and Procedures.

The Company’s management, with the participation of our chief executive officer and chief financial officer, has evaluated the effectiveness of the Company’s disclosure controls and procedures (as defined in Rules

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13a-15(e) or 15d-15(e) under the Securities Exchange Act of 1934, as amended (the “Exchange Act”)) as of the end of the period covered by this report. Based on such evaluation, our chief executive officer and chief financial officer have concluded that, as of the end of such period, the Company’s disclosure controls and procedures were effective in ensuring that information required to be disclosed by the Company in the reports that it files or submits under the Exchange Act is recorded, processed, summarized and reported, on a timely basis, and is accumulated and communicated to the Company’s management, including the Company’s chief executive officer and chief financial officer, as appropriate to allow timely decisions regarding required disclosure.

(b) Management’s Annual Report on Internal Control Over Financial Reporting.

Management of the Company is responsible for establishing and maintaining adequate internal control over financial reporting. Internal control over financial reporting is defined in Rule 13a-15(f) and 15d-15(f) under the Exchange Act as a process designed by, or under the supervision of, the Company’s principal executive and principal financial officers and effected by the Company’s board of directors, management and other personnel to provide reasonable assurance regarding the reliability of financial reporting and the preparation of financial statements for external purposes in accordance with U.S. generally accepted accounting principles and includes those policies and procedures that:

Because of its inherent limitations, internal control over financial reporting may not prevent or detect misstatements. Projections of any evaluation of effectiveness to future periods are subject to the risks that controls may become inadequate because of changes in conditions, or that the degree of compliance with the policies or procedures may deteriorate.

Management assessed the effectiveness of the company’s internal control over financial reporting as of March 31, 2005. In making this assessment, management used the criteria established in Internal Control-Integrated Framework, issued by the Committee of Sponsoring Organizations of the Treadway Commission (COSO). Based on this assessment, our management concluded that, as of March 31, 2005 our internal control over financial reporting is effective based on those criteria. Our management’s assessment of the effectiveness of our internal control over financial reporting as of March 31, 2005 has been audited by Ernst & Young LLP, an independent registered public accounting firm, as stated in their report appearing in Item 9a of this Form 10-K.

/s/ REPLIGEN CORPORATION

May 27, 2005

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(c) Attestation Report of the Independent Registered Public Accounting Firm.

Board of Directors and Stockholders of Repligen Corporation

We have audited management’s assessment, included in the accompanying Report of Management on Internal Control Over Financial Reporting, that the Company maintained effective internal control over financial reporting as of March 31, 2005, based on criteria established in Internal Control-Integrated Framework issued by the Committee of Sponsoring Organizations of the Treadway Commission (the COSO criteria). The Repligen Corporation’s management is responsible for maintaining effective internal control over financial reporting and for its assessment of the effectiveness of internal control over financial reporting. Our responsibility is to express an opinion on management’s assessment and an opinion on the effectiveness of the Company’s internal control over financial reporting based on our audit.

We conducted our audit in accordance with the standards of the Public Company Accounting Oversight Board (United States). Those standards require that we plan and perform the audit to obtain reasonable assurance about whether effective internal control over financial reporting was maintained in all material respects. Our audit included obtaining an understanding of internal control over financial reporting, evaluation of management’s assessment, testing and evaluating the design and operating effectiveness of internal control and performing such other procedures, as we considered necessary in the circumstances. We believe that our audit provides a reasonable basis for our opinion.

A company’s internal control over financial reporting is a process designed to provide reasonable assurance regarding the reliability of financial reporting and the preparation of financial statements for external purposes in accordance with generally accepted accounting principles. A company’s internal control over financial reporting includes those policies and procedures that (1) pertain to the maintenance of records that, in reasonable detail, accurately and fairly reflect the transactions and dispositions of the assets of the Company; (2) provide reasonable assurance that transactions are recorded as necessary to permit preparation of financial statements in accordance with generally accepted accounting principles, and the receipts and expenditures of the company are being made only in accordance with authorizations of management and directors of the Company; and (3) provide reasonable assurance regarding prevention or timely detection of unauthorized acquisition, use, or disposition of the company’s assets that could have a material effect on the financial statements.

Because of its inherent limitations, internal control over financial reporting may not prevent or detect misstatements. Also, projections of any evaluation of effectiveness to future periods are subject to risk that controls may become inadequate because of changes in conditions, or that the degree of compliance with the policies or procedures may deteriorate.

In our opinion, management’s assessment that the Company maintained effective internal control over financial reporting as of March 31, 2005, is fairly stated, in all material respects, based on the COSO criteria. Also, in our opinion, the Company maintained, in all material respects, effective internal control over financial reporting as of March 31, 2005, based on the COSO criteria.

We also have audited, in accordance with the standards the Public Company Accounting Oversight Board (United States), the balance sheets of Repligen Corporation as of March 31, 2005 and 2004, and the related statements of income, shareholders’ equity, and cash flows for each of the three years in the period ended March 31, 2005 and our report dated June 8, 2005, expressed an unqualified opinion thereon.

/s/ Ernst & Young LLP

Boston, Massachusetts

June 8, 2005

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(d) Changes in Internal Control Over Financial Reporting

There have not been any changes in the Company’s internal control over financial reporting (as such term is defined in Rules 13a-15(f) and 15d-15(f) under the Exchange Act) during the quarter ended March 31, 2005 that have material affected, or are reasonably likely to materially affect, the Company’s internal control over financial reporting.

Item 9B. OTHER INFORMATION

The Company’s policy governing transactions in its securities by its directors, officers and employees permits its officers, directors and employees to enter into trading plans in accordance with Rule 10b5-1 under the Exchange Act. The Company has been advised that the following officer has entered into a trading plan during the third quarter of fiscal 2005 in accordance with Rule 10b5-1 and the Company’s policy governing transactions in its securities: Barbara Burnim Day, Vice President. The Company undertakes no obligation to update or revise the information provided herein, including for revision or termination of an established trading plan.

PART III

Pursuant to General Instructions G to Form 10-K, the information required for Part III, Items 10, 11,12, 13 and 14, is incorporated herein by reference from the Company’s proxy statement for the Annual Meeting of Stockholders to be held on September 15, 2005.

PART IV

Item 15. EXHIBITS, FINANCIAL STATEMENT SCHEDULES

The following documents are filed as part of this Annual Report on Form 10-K:

(a) (1) Financial Statements:

The consolidated financial statements required by this item are submitted in a separate section beginning on page F-2 of this Report, as follows:

(a) (2) Financial Statement Schedules:

None

(a) (3) Exhibits:

The Exhibits which are filed as part of this Annual Report or which are incorporated by reference are set forth in the Exhibit Index hereto.

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EXHIBIT INDEX

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The exhibits listed above are not contained in the copy of the Annual Report on Form 10-K distributed to stockholders. Upon the request of any stockholder entitled to vote at the 2005 annual meeting, the Registrant will furnish that person without charge a copy of any exhibits listed above. Requests should be addressed to Repligen Corporation, 41 Seyon Street, Waltham, MA 02453.

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SIGNATURES

Pursuant to the requirements of Section 13 or 15(d) of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

POWER OF ATTORNEY

KNOW ALL MEN BY THESE PRESENTS, that each person whose signature appears below hereby makes, constitutes and appoints Walter C. Herlihy with full power to act without the other, his true and lawful attorney-in-fact and agent, with full power of substitution and resubstitution, for him and in his name, place and stead, in any and all capacities to sign any or all amendments to this Form 10-K, and to file the same with all exhibits thereto, and other documents in connection therewith, with the Securities and Exchange Commission, granting unto said attorney-in-fact and agents, and each of them, full power and authority to do and perform each and every act and thing requisite and necessary to be done in connection therewith, as fully to all intents and purposes as he might or could do in person, hereby ratifying and confirming all that said attorney-in-fact and agents of any of them, or any substitute or substitutes, lawfully do or cause to be done by virtue hereof.

Pursuant to the requirements of the Securities Exchange Act of 1934, this report has been signed below by the following persons on behalf of the registrant and in the capacities and on the dates indicated.

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EXHIBIT INDEX

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34

INDEX TO FINANCIAL STATEMENTS

F-1

REPORT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

The Board of Directors and Stockholders of

Repligen Corporation

We have audited the accompanying balance sheets of Repligen Corporation as of March 31, 2005 and 2004, and the related statements of operations, stockholders’ equity, and cash flows for each of the three years in the period ended March 31, 2005. These financial statements are the responsibility of the Company’s management. Our responsibility is to express an opinion on these financial statements based on our audits.

We conducted our audits in accordance with the standards of the Public Company Accounting Oversight Board (United States). Those standards require that we plan and perform the audit to obtain reasonable assurance about whether the financial statements are free of material misstatement. An audit includes examining, on a test basis, evidence supporting the amounts and disclosures in the financial statements. An audit also includes assessing the accounting principles used and significant estimates made by management, as well as evaluating the overall financial statement presentation. We believe that our audits provide a reasonable basis for our opinion.

In our opinion, the financial statements referred to above present fairly, in all material respects, the financial position of Repligen Corporation as of March 31, 2005 and 2004, and the results of its operations, stockholders’ equity, and cash flows for each of the three years in the period ended March 31, 2005, in conformity with U.S. generally accepted accounting principles.

We have also audited, in accordance with the standards of the Public Company Accounting Oversight Board (United States), the effectiveness of Repligen Corporation’s internal control over financial reporting as of March 31, 2005, based on criteria established in Internal Control-Integrated Framework issued by the Committee of Sponsoring Organizations of the Treadway Commission and our report dated June 8, 2005 expressed in an unqualified opinion thereon.

/s/ ERNST & YOUNG LLP

Boston, Massachusetts

June 8, 2005

F-2

REPLIGEN CORPORATION

BALANCE SHEETS

See accompanying notes.

F-3

REPLIGEN CORPORATION

STATEMENTS OF OPERATIONS