As filed with the Securities and Exchange Commission on November 30, 2007

Registration No. 333-75372

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

POST-EFFECTIVE AMENDMENT NO. 1 TO

FORM S-8

REGISTRATION STATEMENT
UNDER
THE SECURITIES ACT OF 1933

IMMUNOGEN, INC.
(Exact Name of Registrant as Specified in its Charter)

EXPLANATORY NOTE

ImmunoGen, Inc. (the “Company”) is filing this post-effective amendment to deregister certain of the securities originally registered pursuant to the Registration Statement on Form S-8 (File No. 333-75372), filed with the Securities and Exchange Commission on December 18, 2001 by the Company (the “Registration Statement”) with respect to shares of the Company’s common stock, par value $0.01 per share (“Common Stock”), thereby registered for issuance under the Company’s Restated Stock Option Plan, as amended (the “Restated Stock Option Plan”).  An aggregate of 2,500,000 shares of Common Stock were registered for issuance under the Restated Stock Option Plan pursuant to the Registration Statement.

On November 14, 2006, the Company’s shareholders approved the Company’s 2006 Employee, Director and Consultant Equity Incentive Plan (the “2006 Plan”), which replaced the Restated Stock Option Plan as of the date of approval.  Following the approval of the 2006 Plan, no future awards may be made under the Restated Stock Option Plan.  The maximum number of shares of Common Stock reserved and available for issuance under the 2006 Plan includes 500,000 shares available for grant under the Restated Stock Option Plan as of the effective date of the 2006 Plan (the “Original Carried Forward Shares), plus the number of shares underlying any grants previously made under the Restated Stock Option Plan that are forfeited, canceled or terminated (other than by exercise) from and after November 11, 2006.  An aggregate of 811,245 additional shares (the “Additional Carried Forward Shares”) have been included in the shares reserved for issuance under the 2006 Plan as a result of forfeiture, cancellation or termination (other than by exercise) of previously-made grants under the Restated Stock Option Plan during the period between November 11, 2006 and October 31, 2007.  The Original Carried Forward Shares include 111,245 shares registered under the Registration Statement, which are hereby deregistered.  Additional shares reserved for issuance under the Restated Stock Option Plan may become available in the future as a result of the forfeiture, cancellation or termination of awards (other than by exercise) under the Restated Stock Option Plan, and these additional shares will be included in the shares reserved for issuance under the 2006 Plan.

The Original Carried Forward Shares were included in the 2,500,000 shares of Common Stock registered pursuant to the Registration Statement on Form S-8 (File No. 333-138713), filed with the Securities and Exchange Commission on November 15, 2006, for issuance pursuant to the 2006 Plan.  Contemporaneously with the filing of this Post-Effective Amendment No. 1 to the Registration Statement, the Company is filing a Registration Statement on Form S-8 to register the Additional Carried Forward Shares for issuance pursuant to the 2006 Plan.

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SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, the Registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-8 and has duly caused this Post-Effective Amendment No. 1 to the Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the city of Cambridge, the Commonwealth of Massachusetts on this 30th day of November, 2007.

Pursuant to the requirements of the Securities Act of 1933, this Post-Effective Amendment No. 1 to the Registration Statement has been signed by the following persons in the capacities and on the dates indicated.

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EXHIBIT INDEX

* Filed herewith

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Exhibit 23.1

Consent of Independent Registered Public Accounting Firm

We consent to the incorporation by reference in the Post-Effective Amendment No. 1 to the Registration Statement (Form S-8 No. 333-75372) pertaining to the Restated Stock Option Plan of ImmunoGen, Inc. of our reports dated August 28, 2007, with respect to the consolidated financial statements and schedule of ImmunoGen, Inc. included in its Annual Report (Form 10-K) for the year ended June 30, 2007, and the effectiveness of internal control over financial reporting of ImmunoGen, Inc., filed with the Securities and Exchange Commission.