This year has marked a significant transition for ImmunoGen. Two of our partners began clinical testing of products they developed with our Tumor-Activated Prodrug (TAP) technology, we gained back the rights to our product cantuzumab mertansine, and after the close of our fiscal year we established an important collaboration with a major pharmaceutical company, Aventis. ImmunoGen ended the fiscal year with $101 million in cash, and subsequently received a $12 million upfront payment from Aventis.

As a result of these and other successes described in this report, I believe the stage is set for ImmunoGen to move forward in ways not previously possible.

So far, our product accomplishments all derive from our TAP technology, which uses tumor-targeting antibodies to deliver a highly potent cell-killing agent specifically to cancer cells.

In the latter half of 2002, two TAP product candidates developed by our partners advanced into clinical testing: bivatuzumab mertansine by Boehringer Ingelheim and MLN2704 by Millennium Pharmaceuticals. These events triggered milestone payments to ImmunoGen and will help contribute to the expanding body of clinical data on TAP products.

In November 2002, encouraging Phase I findings with the ImmunoGen-developed compound huN901-DM1 (BB-10901) were presented at an international cancer conference, and in December 2002, our partner British Biotech initiated the Phase II portion of this U.S. Phase I/II study. British Biotech also continues to make progress with a Phase I study underway in the United Kingdom with an accelerated dosing regimen.

In January 2003, we ended the negotiations with GlaxoSmithKline that were a result of their desire to change the terms of their license to cantuzumab mertansine. We have regained our rights to cantuzumab mertansine, and we also have rights to all the data from the completed Phase I program. We intend to re-partner cantuzumab mertansine for advancement into the next stages of clinical evaluation and commercialization.

In the past year, we have made important progress with product candidates in our research pipeline, including the TAP product candidate huMy9-6-DM1 for acute myeloid leukemia, our anti-IGF-IR antibody for various solid tumors including lung, breast, and prostate cancers, and our product candidate for certain B-cell malignancies. In July 2003, these product candidates were licensed to Aventis to expedite their development.

Product achievements over the past twelve months reflect the many routes we use to capitalize on our unique technologies and expertise to build value at ImmunoGen.

First, we develop our own products. ImmunoGen developed cantuzumab mertansine, huN901-DM1, and the three compounds licensed to Aventis. ImmunoGen remains committed to developing our own products, which we will commercialize either by ourselves or through partners.

Second, in 2000 we began to out-license our TAP technology to other companies so that they could develop TAP products with their antibodies. This expands the number of TAP product candidates in development and it provides ImmunoGen with a return from all compounds created using our technology. To date, we have received more than $31 million – not including manufacturing payments – through our agreements with Abgenix, Boehringer Ingelheim, Genentech, and Millennium. So far, two partner-developed TAP product candidates have advanced into clinical testing.

Third, this summer we further expanded our opportunities through the formation of a collaboration with Aventis. I believe this collaboration will enable us to develop substantially more products in less time than we could have achieved on our own. ImmunoGen will derive a return from all compounds developed using our technology and expertise. Additionally, the agreement includes significant committed research funding to ImmunoGen. We can now use our cash to further expand our product and technology opportunities.

I believe we are poised to provide increased financial returns for ImmunoGen. Now that we have the Aventis agreement in place, we can take full advantage of our strengths in antibodies and in cancer biology to build our business in ways not previously possible. I look forward to updating you on our progress throughout the year ahead.

Sincerely,
Mitchel Sayare, Ph.D.

Chairman, President, and Chief Executive Officer
September 12, 2003