UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-Q/A

(Amendment No. 2)

x      QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934

For the quarterly period ended March 31, 2012

OR

o         TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934

For the transition period from             to            

Commission file number 0-17999

ImmunoGen, Inc.

830 Winter Street, Waltham, MA 02451

(Address of principal executive offices, including zip code)

(781) 895-0600

(Registrant’s telephone number, including area code)

Indicate by check mark 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.  x Yes  o No

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (§ 232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files).  x Yes  o No

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of “large accelerated filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act. (Check one):

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act).  o Yes  x No

Indicate the number of shares outstanding of each of the issuer’s classes of common stock, as of the latest practicable date.

Shares of common stock, par value $.01 per share:  77,295,075 shares outstanding as of May 1, 2012.

EXPLANATORY NOTE

This Amendment No. 2 to the Quarterly Report on Form 10-Q/A (this “Amendment”) amends the Quarterly Report on Form 10-Q for the quarter ended March 31, 2012 (the “Original Report”) filed by ImmunoGen, Inc. with the Securities and Exchange Commission on May 10, 2012.  This Amendment is being filed solely for the purpose of amending Exhibits 10.2 and 10.3 under Item 6 of Part II of the Original Report in connection with requests for confidential treatment of portions of such exhibits.

This Amendment continues to speak as of May 10, 2012, the date of the Original Report, and except as described above, no other changes have been made to the Original Report and this Amendment does not modify or update disclosures in the Original Report and does not reflect subsequent events occurring after the date of the Original Report.  Accordingly, this Amendment should be read in conjunction with the Original Report.

PART II. OTHER INFORMATION

ITEM 6.                        Exhibits

*                                         Portions of this Exhibit were omitted, as indicated by [***], and have been filed separately with the Secretary of the Commission pursuant to the Registrant’s application requesting confidential treatment.

**                                  Previously filed with our Quarterly Report on Form 10-Q on May 10, 2012 which this Form 10-Q/A amends.

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SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, as amended, the Registrant has duly caused this report to be signed on its behalf by the undersigned thereunto duly authorized.

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