Biogen Reconfirms Full Year 2003 Revenue and EPS Guidance and Announces Revised Q2 EPS Outlook

Full Year Operating EPS Reconfirmed at $1.72 to $1.85 As Stronger Avonex Sales Expected to Offset Lower Royalty Revenue Second Quarter 2003 EPS Adjusted to $0.32 to $0.38 Reflecting Softer Royalties from Schering-Plough’s INTRON franchise

Biogen, Inc. (NASDAQ: BGEN) today reconfirmed operating earnings per share guidance for full year 2003 of $1.72 to $1.85. Biogen reiterated previous guidance that it expects quarterly earnings to be uneven during 2003. In addition, as discussed at Biogen’s June 6, 2003 annual stockholder meeting, the Company believes that full year royalty revenue will be at the low end of the guidance range, due primarily to lower royalties from Schering-Plough Corporation as a result of reductions in trade inventory levels affecting its INTRON® A (interferon alpha-2b) product. Biogen now believes that most of this impact will be in the second quarter of 2003. Largely as a result of this impact, Biogen expects second quarter earnings to be in the $0.32 to $0.38 per share range.

REGULATION G STATEMENT – GAAP FINANCIAL MEASURES

Guidance for full-year 2003 reported earnings per share, which is the most comparable GAAP-based financial measure to operating earnings per share, is not currently accessible as Biogen cannot predict with any certainty the nature or the amount of non-operating or unusual charges for subsequent quarters. Biogen does, however, anticipate that it may have to take such charges in subsequent quarters and that such charges, if material, would cause reported earnings per share to differ from operating earnings per share.

Biogen also announced today it signed a definitive merger agreement with IDEC Pharmaceuticals Corp. Details are available in a separate press release.

ABOUT BIOGEN

Biogen is the world’s oldest independent biotechnology company and a leader in biologics research, development and manufacturing. A pioneer in leading edge research in immunology, neurobiology and oncology, Biogen brings novel therapies to improve patients’ lives around the world through its global marketing capabilities. For press releases and additional information about the company, please visit http://www.biogen.com.

FORWARD LOOKING STATEMENTS / SAFE HARBOR

This press release contains forward-looking statements regarding expected future financial results.

These statements are based on the Company’s current beliefs and expectations. A number of risks and uncertainties could cause actual results to differ materially. For example, financial results, including earnings per share, sales, and expense amounts, may be affected by any slowing of growth of the multiple sclerosis market, any change in market acceptance of AVONEX in key markets worldwide, the Company’s ability to achieve market acceptance of AMEVIVE in the U.S., the impact of reimbursement and pricing decisions related to the Company’s products, the impact of competitive products on AVONEX and AMEVIVE sales, the impact of litigation, any unanticipated increase in costs and expenses including in the areas of research and development, manufacturing, and sales and marketing, in-licensing and product opportunities and factors affecting sales of licensee’s products that may impact royalty revenues.

For more detailed information on the risks and uncertainties associated with these forward looking statements and the Company’s other activities see the Outlook section in MD&A of the Company’s Annual Report on Form 10-K and quarterly reports on Form 10-Q filed with the Securities and Exchange Commission. The Company does not undertake any obligation to publicly update any forward-looking statements.