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1 Filed Pursuant to Rule 424(b)(3) REGISTRATION NO. 333-07661 PROSPECTUS SUPPLEMENT NO. 1 DATED JULY 18, 1996 TO THE PROSPECTUS DATED JULY 12, 1996 OF IMMUNOGEN, INC. This Prospectus Supplement No. 1 amends the information in the section entitled "Selling Stockholders" in the Prospectus dated July 12, 1996. Dana-Farber has entered into an arrangement with Prudential Securities Incorporated ("Prudential") whereby some or all of the 351,662 shares of common stock of ImmunoGen, Inc., $.01 par value per share, issued to Dana-Farber upon conversion of the Debenture may be sold to Prudential as principal or through Prudential acting as Dana-Farber's agent. In order to reflect Prudential as a potential Selling Shareholder hereunder, and assuming for such purposes that all shares are sold to Prudential as principal, the Selling Stockholder table is amended as follows:
--------------------------- This Prospectus Supplement No. 1 should be read in conjunction with the Prospectus dated July 12, 1996. Page 1 of 1 PageShares Owned Prior to Shares Owned After Offering Offering(1) ------------------- Number of Shares ---------------- Selling Stockholder Number Percent Being Offered Number Percent - ------------------- ------ ------- ---------------- ------ ------- Dana-Farber 351,662(2) 2.1% 351,662 0 -- Prudential Securities 351,662(3) 2.1% 351,662 0 -- Incorporated LBC 250,000(4) 1.5% 250,000 0 -- - ------------------------ (1) Assumes the sale of all shares offered hereby to unaffiliated third parties. (2) Based on 16,599,855 shares of Common Stock outstanding on June 30, 1996, and adjusted to reflect the conversion by Dana-Farber of the Debenture into 351,662 shares of Common Stock. (3) Assumes purchase by Prudential Securities Incorporated as principal of the 351,662 shares of Common Stock issued to Dana-Farber upon conversion of the Debenture. (4) Based on 16,599,855 shares of Common Stock outstanding on June 30, 1996, and adjusted to reflect the exercise of the LBC Warrants.