As I write this Letter, I am struck by how much we achieved this year. We now have two TAP compounds in clinical testing – both wholly-owned by ImmunoGen – along with collaborations with seven other companies. Four companies either took licenses and/or expanded their relationship with us this year. And two collaborators have TAP compounds in clinical testing and we expect still more products to follow these.

We believe our Tumor-Activated Prodrug (TAP) technology has the potential to improve the lives of cancer patients, expand the opportunity for antibodies in the treatment of cancer, and provide significant returns to our shareholders.

Our HuN901-DM1 Anticancer Compound

In early 2004, we regained the rights to our huN901-DM1 compound from our former partner British Biotech (now Vernalis) and in mid-2004 we took control of its development.

Since then, we have invigorated its evaluation for small-cell lung cancer (SCLC) and expanded its clinical program to include a new indication. After taking over the weekly-dosing SCLC Phase I/II study that had been underway for some time, we increased the rate of patient enrollment, presented encouraging initial findings – including reports of measurable tumor shrinkage – and expanded the study to include more patients. Our goal is to complete most of the patient enrollment in this study in the next twelve months.

Patient enrollment also has increased in the other study underway with huN901-DM1 in the treatment of SCLC – a dose-escalation Phase I trial in which the compound is administered daily for three days in a twenty-one-day cycle. We expect to report findings from this study in the next few months.

Interestingly, the antigen that huN901-DM1 targets, CD56, is found on certain hematologic, or “liquid,” tumors as well as on SCLC and this provides us with another – and potentially faster – development pathway for the compound. Based on favorable preclinical findings, we have initiated a Phase I study with huN901-DM1 in multiple myeloma. Our goal is to complete much of the patient enrollment in this study in the next twelve months.

Our HuC242-DM4 Anticancer Compound

In June 2005, we put our second TAP compound, huC242-DM4, into clinical testing for the treatment of CanAg-expressing cancers, such as colorectal and pancreatic. HuC242-DM4 includes the same huC242 antibody as our earlier compound, cantuzumab mertansine, but incorporates a different cell-killing agent, DM4. Based on our preclinical results, we expect huC242-DM4 to have significantly greater clinical efficacy than cantuzumab mertansine, with a similar safety profile. We will keep you posted on our progress.

Collaborator Developments

To expand the application of our TAP technology – and help us fund our own product programs – we selectively outlicense our TAP technology to other companies.

In late 2004, two leading antibody companies – Biogen Idec and the Centocor subsidiary of Johnson & Johnson – each licensed the right to use our TAP technology with antibodies to undisclosed targets. Additionally, in recent months, another leading antibody company – Genentech – took their second and third licenses to our TAP technology. Genentech now has rights to use our technology with antibodies to two undisclosed targets as well as to HER2. Additionally, in May 2005, Genentech renewed the agreement that enables them to test our technology with additional antibodies.

We were disappointed in February 2005 when Boehringer Ingelheim had to discontinue development of their bivatuzumab mertansine TAP compound because of a side effect – skin toxicity – not seen with any other TAP compound in clinical testing. The CD44v6 target for bivatuzumab mertansine occurs on normal proliferating skin cells as well as on certain cancers. However, Boehringer Ingelheim retained the right to use our TAP technology with antibodies to a different target and recently exercised this right.

In May 2005, another of our licensees, Millennium Pharmaceuticals, Inc., reported favorable initial Phase I/II data for their TAP compound, MLN2704, which is in development for the treatment of prostate cancer. In early clinical testing, MLN2704 has demonstrated promising evidence of anticancer activity, including measurable tumor shrinkage and reductions in PSA, a biomarker associated with prostate cancer progression.

In 2003, we entered into a research, development, and commercialization agreement with Aventis that was then assumed by the sanofi-aventis Group in 2004. We are pleased to report that the merger of Aventis and Sanofi-Synthelabo has had little impact on our collaboration. In fact, in March 2005, the sanofi-aventis Group advanced the lead collaboration compound, AVE9633 (huMy9-6-DM4), into clinical testing and, in August 2005, the sanofi-aventis Group elected to extend the duration of our joint research collaboration to include a fourth year. Among other things, this means that we will receive another $18.2 million starting September 2006, on top of the $50.7 million we began to receive in September 2003. And our scientists will continue to work together to develop compounds that potentially can generate revenue for both companies.

Every product developed by us or by our collaborators has the potential to yield data that demonstrate the significance of our TAP technology. Indeed, through our programs and those of our partners, the body of data that supports our TAP technology should expand considerably in the not-too-distant future.

Our Company

In May 2005, we further strengthened our management team with the addition of Daniel Junius as our Chief Financial Officer. And in June 2005, we added Nicole Onetto, M.D. – an oncologist – to our Board, reflecting the increased importance of clinical development to ImmunoGen.

We ended our 2005 fiscal year with $91 million in cash and marketable securities and have no debt. In our 2006 fiscal year we expect to increase our financial commitment to our own compounds, since we strongly believe that the best use of our cash is for us to develop our own products and rapidly demonstrate the value of our technology.

We are excited about what lies ahead and look forward to updating you on our progress.

Sincerely,

Mitchel Sayare, Ph.D.
Chairman, President, and Chief Executive Officer
September 9, 2005