UNITED STATES
SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

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

Date of Report (Date of earliest event reported): August 30, 2005

ImmunoGen, Inc.

(Exact name of registrant as specified in its charter)

128 Sidney Street, Cambridge, MA 02139

(Address of principal executive offices)   (Zip Code)

Registrant’s telephone number, including area code: (617) 995-2500

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

o                                    Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

o                                    Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

o                                    Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

o                                    Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

ITEM 8.01 – OTHER EVENTS

On August 31, 2005, ImmunoGen, Inc. (Nasdaq: IMGN) announced that Aventis Pharmaceuticals, Inc. of the sanofi-aventis Group has exercised its right to extend the term of its research collaboration with the Company, and committed to pay ImmunoGen a minimum of $18.2 million in research support over the twelve months beginning September 1, 2006. This funding is in addition to the $50.7 million in research support already committed for the three-year period ending August 31, 2006.

A copy of the press release is attached to this current report on Form 8-K as Exhibit 99.1.

ITEM 9.01. FINANCIAL STATEMENTS AND EXHIBITS

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SIGNATURES

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

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Exhibit 99.1

Contacts:

For Immediate Release

ImmunoGen, Inc. Announces that the sanofi-aventis Group Has Extended its
Research Collaboration with the Company

– Extension of Research Collaboration Begins September 2006
and Provides ImmunoGen with Additional Committed Funding –

CAMBRIDGE, MA, August 31, 2005 – ImmunoGen, Inc. (Nasdaq: IMGN) today announced that Aventis Pharmaceuticals Inc. of the sanofi-aventis Group has exercised its right to extend the term of its research collaboration with the Company and committed to fund ImmunoGen $18.2 million in research support over the twelve months beginning September 1, 2006. This funding is in addition to the research support already committed for the three years ending August 31, 2006.

Mitchel Sayare, PhD, Chairman and CEO, commented, “We are delighted that the sanofi-aventis Group has elected to extend this research collaboration. The extension enables the two companies to work together for a fourth year to develop new compounds that can potentially generate revenue for ImmunoGen.”

In July 2003, ImmunoGen and Aventis Pharmaceuticals, Inc. established a collaboration to discover, develop, and commercialize novel antibody-based anticancer products, an agreement that was maintained by the sanofi-aventis Group subsequent to Sanofi-Synthelabo’s acquisition of Aventis in 2004. The agreement included an upfront payment and committed research support funding to ImmunoGen for the three-year period beginning September 1, 2003. The collaboration agreement also entitles ImmunoGen to milestone payments and royalties on compounds that result from the research collaboration, as well as manufacturing payments for material produced for the sanofi-aventis Group.

The 2003 agreement allows the sanofi-aventis Group to extend the term of the research collaboration for two additional one-year periods – the year beginning September 1, 2006, for which the extension right now has been exercised, and the year beginning September 1, 2007 – by providing ImmunoGen with one year of notice for each extension.

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About ImmunoGen, Inc.

ImmunoGen, Inc. develops targeted anticancer biopharmaceuticals. The Company’s proprietary Tumor-Activated Prodrug (TAP) technology uses tumor-targeting antibodies to deliver a potent, cell-killing agent specifically to cancer cells. Four TAP compounds are in clinical testing – huN901-DM1 and huC242-DM4, which are wholly owned by ImmunoGen, and MLN2704 and AVE9633, which are in development by ImmunoGen collaborators. Genentech, Centocor (a wholly-owned subsidiary of Johnson & Johnson), Biogen Idec, the sanofi-aventis Group, Millennium Pharmaceuticals, Inc., Boehringer Ingelheim, and Abgenix have licensed the right to develop and/or test TAP compounds to specific targets; ImmunoGen also has a broader collaboration with the sanofi-aventis Group.

This press release includes forward-looking statements. For these statements, ImmunoGen claims the protection of the safe harbor for forward-looking statements provided by the Private Securities Litigation Reform Act of 1995. It should be noted that there are risks and uncertainties related to the development of collaboration products, as well as the Company’s development of its own products. A review of these risks can be found in ImmunoGen’s Annual Report on Form 10-K for the fiscal year ended June 30, 2005 and other reports filed with the Securities and Exchange Commission.

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