Exhibit 4.1

AMENDMENT TO RIGHTS AGREEMENT

        THIS AMENDMENT (the "Amendment"), dated as of the 19th day of July, 2011, to the Rights Agreement (the "Rights Agreement"), dated May 6, 2003, as amended on March 4, 2005, January 29, 2007 and July 17, 2009, between ALLOS THERAPEUTICS, INC., a Delaware corporation (the "Company"), and MELLON INVESTOR SERVICES LLC (the "Rights Agent"), is being executed at the direction of the Company and shall be effective immediately prior to the Company's entry into that certain Agreement and Plan of Merger and Reorganization (as it may be amended from time to time, the "Merger Agreement") to be entered into by and among the Company, AMAG Pharmaceuticals, Inc., a Delaware corporation ("Parent"), and Alamo Acquisition Sub, Inc., a Delaware corporation and wholly-owned subsidiary of Parent; provided, however, if (i) the Merger Agreement is not executed as of even date herewith, or (ii) the Merger Agreement terminates or is terminated in accordance with the terms thereof prior to the Effective Time (as defined therein), this Amendment shall terminate, in either such case, immediately upon the Rights Agent's receipt of written notice thereof from the Company, and shall be of no further force and effect. Capitalized terms used in this Amendment and not otherwise defined herein shall have the meanings given them in the Rights Agreement.

        WHEREAS, Section 27 of the Rights Agreement provides that the Company and the Rights Agent shall, if the Company so directs, supplement or amend any provision of the Rights Agreement without the approval of any holders of the Rights, any such supplement or amendment to be evidenced by a writing signed by the Company and the Rights Agent; and

        WHEREAS, pursuant to Section 27 of the Rights Agreement, the Company has delivered to the Rights Agent a certificate signed by an authorized officer of the Company certifying that the proposed amendment of the Rights Agreement is in compliance with the terms of Section 27 of the Rights Agreement.

        NOW, THEREFORE, in consideration of the foregoing and the agreements, provisions and covenants herein contained, the parties agree as follows:

1.

Section 1(h) shall be amended and restated in its entirety as follows:

"Excluded Stockholder" shall include each of (i) Warburg Pincus Private Equity VIII, L.P. (including its Affiliates and Associates) ("Warburg"); provided, however, that Warburg shall not be an Excluded Stockholder if, without the prior approval of the Board of Directors, Warburg becomes the Beneficial Owner of more than 44% of the outstanding Common Shares; (ii) Baker Brothers Life Sciences, L.P. (including its Affiliates and Associates) ("Baker"); provided, however, that Baker shall not be an Excluded Stockholder if, without the prior approval of the Board of Directors, Baker becomes the Beneficial Owner of more than 20% of the outstanding Common Shares, and (iii) Parent, its stockholders, Merger Sub and their respective Affiliates and Associates to the extent that any of them would otherwise become an Acquiring Person solely as the result of (x) the execution, delivery or performance of the Merger Agreement, the Parent Stockholder Voting Agreements or the Company Stockholder Voting Agreements, (y) their acquisition or their right to acquire beneficial ownership of the Common Shares as a result of their execution of the Merger Agreement, or (z) the consummation of the Merger or any of the other transactions contemplated by the Merger Agreement, the Company Stockholder Voting Agreements and the Parent Stockholder Voting Agreements.

2.

Section 1 shall be amended by adding the following new Section 1(s) at the end thereof:

        (s)   The following additional terms shall have the meanings indicated:

          (i)  "Parent" shall mean AMAG Pharmaceuticals, Inc., a Delaware corporation.

         (ii)  "Parent Stockholder Voting Agreement" shall mean each of the voting agreements in favor of the Company to be executed concurrently with the execution of the Merger Agreement by certain stockholders of Parent.

       (iii)  "Company Stockholder Voting Agreement" shall mean each of the voting agreements in favor of Parent to be executed concurrently with the execution of the Merger Agreement by certain stockholders of the Company.

        (iv)  "Effective Time" shall mean such time as the Merger shall have become effective pursuant to the filing of a certificate of merger with the Secretary of State of the State of Delaware in accordance with the Merger Agreement.

         (v)  "Expiration Date" shall have the meaning set forth in Section 7(a) hereof.

        (vi)  "Merger" shall mean the merger of Merger Sub with and into the Company pursuant to and in accordance with the terms of the Merger Agreement.

       (vii)  "Merger Agreement" shall mean that certain Agreement and Plan of Merger and Reorganization to be entered into by and among the Company, Parent and Merger Sub, as amended from time to time in accordance with the terms thereof.

      (viii)  "Merger Sub" shall mean Alamo Acquisition Sub, Inc., a Delaware corporation and wholly-owned subsidiary of Parent.

3.

Section 7(a) shall be amended and restated in its entirety as follows:

The registered holder of any Right Certificate may exercise the Rights evidenced thereby (except as otherwise provided herein) in whole or in part at any time after the Distribution Date upon surrender of the Right Certificate, with the form of election to purchase on the reverse side thereof duly executed, to the Rights Agent at the office of the Rights Agent designated for such purpose, together with payment of the Purchase Price for each one one-hundredth of a Preferred Share (or such other number of shares or other securities) as to which the Rights are exercised, at or prior to the earliest of (i) the Close of Business on May 28, 2013 (the "Final Expiration Date"), (ii) the time at which the Rights are redeemed as provided in Section 23 hereof (the "Redemption Date"), (iii) the time at which such Rights are exchanged as provided in Section 24 hereof, or (iv) the time immediately prior to the Effective Time (but subject to the occurrence of the Effective Time) (the earlier of (i), (ii), (iii) and (iv) being herein referred to as the "Expiration Date"), whereupon the Rights shall expire and this Agreement shall terminate in accordance with Section 36.

4.

Section 26 of the Rights Agreement is hereby amended by replacing the addresses of the Rights Agent with the following addresses:

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