Immunogen (IMGN) 10-K 2004 FY Annual report

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EXHIBIT 23

Consent of Registered Public Accounting Firm

        We consent to the incorporation by reference in the Registration Statements on Form S-3 (Nos. 333-02441, 333-07661, 333-15819, 333-22153, 333-31795, 333-48042, 333-48385, 333-57234 and 333-100123) and in the related Prospectuses and on Form S-8 (Nos. 333-41534, 333-73544, 333-47543, 333-53292, 333-75372, and 333-75374) pertaining to the ImmunoGen, Inc. Restated Stock Option Plan and the ImmunoGen, Inc. 2001 Non-Employee Director Stock Plan of our report dated July 26, 2004, with respect to the consolidated financial statements and financial statement schedule of ImmunoGen, Inc. included in its Annual Report (Form 10-K) for the year ended June 30, 2004.

/s/  ERNST & YOUNG LLP      

Boston, Massachusetts
August 17, 2004

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EXHIBIT 23