BIOGEN REAFFIRMS FIRST QUARTER AND FULL YEAR 2001 GUIDANCE WITH UPDATE ON ROYALTY REVENUE STATUS

Biogen today confirmed its earnings guidance provided in its year end conference call. At that time, Biogen indicated that earnings per share would be between $0.45 – 0.46 for the first quarter of this year and in the low $1.90s per share for the full year 2001. As was noted in the year end conference call, this guidance took into account several variables including improved product revenue trends and reduced royalty revenue.

Biogen expects full year royalty revenue for 2001 to be in the range of $85 – 95 million as a result of patent expirations, other patent-related events and an ongoing dispute with Schering-Plough on its royalty obligations. The impact of these events on royalty revenue was included in the earnings range given by the Company in January.

Peter Kellogg, Biogen’s Chief Financial Officer confirmed, “We are on track to deliver strong results this quarter. Product revenues continue to be robust. AVONEX? (Interferon beta-1a) remains the number one treatment for Multiple Sclerosis now with approximately 100,000 patients worldwide. In addition, we continue to make significant progress towards our clinical pipeline goal of doubling the number of compounds in clinical development this year. We will begin Phase III trials for ANTEGREN? (natalizumab) in Multiple Sclerosis and Crohn’s disease this year and are looking forward to reporting results from our Phase III trials for AMEVIVE? (alefacept) in moderate-severe chronic plaque psoriasis by mid-year. We are excited about this unprecedented level of Phase III activity at Biogen.”

In addition to historical facts, this press release contains forward-looking statements that involve risks and uncertainties that could cause actual results to differ materially from those reflected in the forward-looking statements. Reference is made in particular to statements regarding the anticipated range of revenues and earnings per share for the quarter and the year and the Company’s expectations regarding the commencement of clinical trials and reporting of clinical data. Factors which could cause actual financial results to differ from the Company’s current expectations include, without limitation, the impact of competitive products on AVONEX® (Interferon beta-1a) sales, any change in market acceptance for AVONEX in key markets worldwide, any unexpected negative results related to AVONEX, and any unanticipated increase in expenses including in the areas of research and development, litigation and in-licensing opportunities and the other risks and uncertainties associated with drug development and commercialization described in the Company’s periodic reports filed with the Securities and Exchange Commission. Factors which could cause actual results regarding clinical trials and reporting of clinical data to differ from the Company’s current expectations include the risk that the Company will encounter one or more technical hurdles associated with new drug development or that problems or delays may arise during preparations for or the conduct of clinical trials, as well as the other risks and uncertainties associated with drug development described in the Company’s periodic reports filed with the Securities and Exchange Commission. Drug development involves a high degree of risk. Success in early stage clinical trials or preclinical work does not ensure that later stage or larger scale clinical trials will be successful.

Biogen, Inc., winner of the U.S. National Medal of Technology, is a biopharmaceutical company principally engaged in discovering and developing drugs for human healthcare through genetic engineering. Headquartered in Cambridge, MA, the Company’s revenues are generated from international sales of AVONEX® (Interferon beta-1a) for treatment of relapsing forms of multiple sclerosis, and from the worldwide sales by licensees of a number of products, including alpha interferon and hepatitis B vaccines and diagnostic products ( Prescribing Information ). Biogen’s research and development activities are focused on novel products to treat inflammatory and autoimmune diseases, neurological diseases, cancer, fibrosis and congestive heart failure. The Company maintains active clinical research programs in protein therapeutics, small molecules, genomics and gene therapy. For copies of press releases and additional information about the Company, please consult Biogen’s Homepage on the World Wide Web at http://www.biogen.com