<PAGE>   1


                UNITED STATES SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                    FORM 10-K

            [X] ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE
                         SECURITIES EXCHANGE ACT OF 1934
                   For the fiscal year ended December 31, 2000

                                       OR

          [ ] TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE
                         SECURITIES EXCHANGE ACT OF 1934

                         Commission file number: 0-12042

                                  BIOGEN, INC.
             (Exact name of Registrant as specified in its charter)


           Massachusetts                                      04-3002117
  (State or other jurisdiction                             (I.R.S. Employer
of incorporation or organization)                         Identification No.)


               14 Cambridge Center, Cambridge, Massachusetts 02142
               (Address of principal executive offices) (zip code)

       Registrant's telephone number, including area code: (617) 679-2000

        Securities registered pursuant to Section 12(b) of the Act: None

           Securities registered pursuant to Section 12(g) of the Act:
                          Common Stock, $.01 par value
                                (Title of class)


     Indicate by check mark whether the Registrant (1) has filed all reports
required to be filed by Section 13 or 15(d) of the Securities Exchange Act of
1934 during the preceding 12 months (or for such shorter period that the
Registrant was required to file such reports), and (2) has been subject to such
filing requirements for the past 90 days.

                                  Yes  X   No
                                      ---     ---

     Indicate by check mark if disclosure of delinquent filers pursuant to Item
405 of Regulation S-K is not contained herein, and will not be contained, to the
best of Registrant's knowledge, in definitive proxy or information statements
incorporated by reference in Part III of this Form 10-K or any amendment to this
Form 10-K. [ ]

     Aggregate market value of Common Stock held by nonaffiliates of the
Registrant at March 29, 2001 (excludes shares held by directors):
$9,485,799,873. Exclusion of shares held by any person should not be construed
to indicate that such person possesses the power, direct or indirect, to direct
or cause the direction of management or policies of the Registrant, or that such
person is controlled by or under common control with the Registrant. Common
Stock outstanding at March 29, 2001: 150,270,097 shares.

                       DOCUMENTS INCORPORATED BY REFERENCE

     Portions of the Registrant's definitive Proxy Statement for its 2001 Annual
Meeting of Stockholders are incorporated by reference into Part III of this
Report, and portions of the Registrant's 2000 Annual Report to Shareholders are
incorporated by reference into Parts II and IV of this Report.

<PAGE>   2



PART I

ITEM 1 - BUSINESS

OVERVIEW

     Biogen, Inc. ("Biogen" or the "Company") is a biopharmaceutical company
principally engaged in the business of developing, manufacturing and marketing
drugs for human health care. Biogen, which was founded in 1978, currently
derives revenues from sales of its AVONEX(R) (Interferon beta-1a) product for
the treatment of relapsing forms of multiple sclerosis and from royalties on
worldwide sales by the Company's licensees of a number of products covered under
patents controlled by the Company. Such products include certain forms of alpha
interferon, hepatitis B vaccines and hepatitis B diagnostic test kits, among
others. The Company's revenues from sales of AVONEX(R) in 2000 were
approximately $761.1 million, making AVONEX(R) the worldwide market leader among
multiple sclerosis therapies. The Company's royalty revenues in 2000 were
approximately $165.4 million.

     Biogen continues to have an active development program related to
AVONEX(R), and is conducting several important clinical trials of the product.
In 2000, Biogen announced the results of its CHAMPS trial, begun in 1996, to
study the effect of AVONEX(R) in delaying the development of clinically definite
multiple sclerosis in patients who had experienced an isolated, well-defined
neurologic event consistent with MS. The study showed a highly statistically
significant beneficial effect of AVONEX(R) on delaying the development of
clinically definite multiple sclerosis as compared to the placebo. A follow-on
open label trial is ongoing. In 2000, Biogen also completed a small Phase 2
pilot study of the use of AVONEX(R) in the treatment of patients with primary
progressive multiple sclerosis. In the study the patient group treated with
AVONEX(R) experienced a significant reduction in the size of brain lesions seen
on MRI, a measure of the extent of damage the disease has caused to the brain,
as compared to the placebo group. In January 2001, Biogen announced preliminary
results of a study of the use of AVONEX(R) in the treatment of patients with
secondary progressive multiple sclerosis. In the study the patient group treated
with AVONEX(R) showed a statistically significant reduction in the rate of
disability accumulation compared to the placebo group as measured by the
multiple sclerosis functional composite (MSFC) endpoint.

     Biogen also continues to devote significant resources to its other ongoing
development efforts. In 2000, the Company completed dosing in its Phase 3
clinical studies of its AMEVIVE(TM) (alafacept) product in patients with
moderate to severe chronic plaque psoriasis. Biogen anticipates that the results
of these studies will be made available during the second quarter of 2001. In
January 2001, the Company announced positive results from two large Phase 2
studies of the use of Antegren(R) (natalizumab) in the treatment of multiple
sclerosis and Crohn's Disease. Antegren(R) is being developed in a collaboration
between Biogen and Elan Corporation, plc ("Elan"). Based on the Phase 2 results,
Biogen and Elan plan to proceed with Phase 3 clinical studies of Antegren(R) in
both MS and Crohn's Disease in 2001. Biogen also plans to begin Phase 1 clinical
trials of several products in 2001, including its small molecule adenosine A1
receptor antagonist which is being developed as a potential treatment for
congestive heart failure and its soluble lymphotoxin beta receptor Ig fusion
protein being developed as a potential treatment for several autoimmune
diseases.

                                       2

<PAGE>   3

     In addition to its development programs, Biogen also has many earlier-stage
research programs. Biogen's research strategy is to direct its efforts toward
finding therapeutics in four major research focus areas: Fibrosis, oncology,
immunomodulation, and neurodegeneration. The Company believes that this focused
research effort along with the leveraging of its traditional strengths in
biologics research, protein chemistry and protein manufacturing will allow the
Company to be in a position to capitalize on the potential of the genomics era.
The Company is exploring the use of functional genomics tools and technology to
find novel therapeutics through its collaborations with Eos Biotechnology, Inc.,
Lexicon Genetics Incorporated, MorphoSys AG, Gene Logic, Inc., Incyte Genomics,
Inc. and Genetica Incorporated.

AVONEX(R) (INTERFERON BETA-1A)

     Biogen currently markets and sells AVONEX(R) (Interferon beta-1a) for the
treatment of relapsing forms of multiple sclerosis. Multiple sclerosis is a
progressive neurological disease in which the body loses the ability to transmit
messages along nerve cells, leading to a loss of muscle control, paralysis and,
in some cases, death. Patients with active relapsing multiple sclerosis
experience an uneven pattern of disease progression characterized by periods of
stability interrupted by flareups of the disease after which the patient returns
to a new baseline of functioning. AVONEX(R) is a recombinant form of a protein
produced by fibroblast cells in response to viral infection. AVONEX(R) has been
shown in a pivotal clinical trial both to slow the accumulation of disability
and to reduce the frequency of exacerbations in patients with relapsing forms of
multiple sclerosis.

     Biogen began selling AVONEX(R) in the United States in 1996, and in the
European Union ("EU") in 1997. AVONEX(R) is on the market in over 50 countries,
including Argentina, Australia, Brazil, Canada, Chile, Columbia, Cyprus, the
Czech Republic, the countries of the EU, Hungary, Israel, Mexico, Norway,
Slovakia, South Africa, Switzerland, Turkey and the United States.

     In the United States, Canada and most of the major countries of the EU,
Biogen uses its own sales force to market AVONEX(R). In those countries, Biogen
distributes AVONEX(R) principally through wholesale distributors of
pharmaceutical products, mail order, specialty distributors or shipping service
providers. In other countries, Biogen sells AVONEX(R) to distribution partners
who are then responsible for most marketing and distribution activities. The
Company has entered into distribution agreements covering Australia, Eastern
Europe, Greece, Israel, Italy, Japan, Latin America, the Middle East, New
Zealand, Portugal, South Africa, Spain and Turkey. Under most of these
agreements, the distribution partners are responsible for marketing and
distributing AVONEX(R).

     Biogen continues to have an active development program related to
AVONEX(R). In February 2000, Biogen announced results of a clinical study of
AVONEX(R) in patients who had experienced only one confirmed demyelinating event
(multiple sclerosis-type exacerbation). The study, called the Controlled High
Risk AVONEX(R) Multiple Sclerosis Prevention Study (CHAMPS), which was initiated
in 1996, showed a highly statistically significant beneficial effect of
AVONEX(R) in delaying the development of clinically definite multiple sclerosis
compared to placebo. The CHAMPS study was stopped early following positive
results. Biogen also completed in 2000 a dose comparison study, initiated in
1996, comparing the approved dosage of AVONEX(R) with a higher dose. The study
confirmed that the higher dose provided no additional benefit as compared to the
currently marketed 30 mcg dose, and that the two doses were equally effective in
reducing disability progression in patients with relapsing/remitting MS. Biogen
also completed in 2000 a small Phase 2 pilot study of AVONEX(R)


                                       3
<PAGE>   4


in the treatment of primary progressive multiple sclerosis. In the study, the
patient group treated with AVONEX(R) experienced a significant reduction in the
size of brain lesions seen on MRI as compared to the placebo group. In 2001,
Biogen announced completion of a double blinded, randomized clinical study of
AVONEX(R) in patients with secondary progressive multiple sclerosis. In the
study, initiated in 1998, treatment with AVONEX(R) was shown to reduce the
progression of disability in secondary progressive MS by 27% as compared to
treatment with the placebo and as measured by the multiple sclerosis functional
composite (MSFC) endpoint.

     Biogen is currently conducting several other clinical studies of AVONEX(R).
These include: an open-label follow-up study initiated in 1995 to obtain
long-term safety and antigenicity data and an open label follow-up study
initiated in 2000 to study the long-term effect of AVONEX(R) on patients who
participated in the CHAMPS study.

     Biogen is also exploring new ways to improve the formulation and delivery
of AVONEX(R). In February 1999, Biogen entered into a collaborative agreement
with Inhale Therapeutic Systems, Inc. under which the parties are working
towards development of a dry powder formulation of AVONEX(R) for pulmonary
delivery using Inhale's deep-lung delivery system. Biogen is also continuing to
work towards development of a pre-filled syringe formulation of AVONEX(R).

     Revenues from sales of AVONEX(R) in 2000 were $761.1 million or
approximately 82% of total revenues. Revenues from sales of AVONEX(R) in 1999
and 1998 were $620.6 million and $394.9 million, respectively, or approximately
78% and 71% of total revenues, respectively. Approximately 73% of AVONEX(R)
sales in 2000, 71% of AVONEX(R) sales in 1999 and 77% of AVONEX(R) sales in 1998
were generated in the United States. Sales to three major wholesale distributors
and a specialty distributor in the United States accounted for 18%, 13%, 12% and
11%, respectively, of total revenues in 2000.

MAJOR RESEARCH AND DEVELOPMENT PROGRAMS

     Biogen's research is focused on biological systems and processes where its
scientific expertise in molecular biology, cell biology, immunology and protein
chemistry can lead to a greater understanding of disease processes and, as a
result, to the creation of new pharmaceuticals. Biogen selects product
candidates from its research programs to test in clinical trials, focusing its
efforts on those agents which it believes have the greatest potential
competitive advantages and large commercial markets. Described below are
Biogen's major research programs.

         AMEVIVE(TM)  (ALAFACEPT)

     Inflammation is the result of the body's immune response to infection and
injury. In many autoimmune diseases, the inflammation process is directed
inappropriately against the body's own tissues, causing temporary or permanent
damage. Biogen has focused the efforts of its inflammation programs on
developing drugs to inhibit specific cellular interactions critical to the
inflammation process. Central to inflammation is the activation of T-cells,
specialized white blood cells which initiate and control the immune response.
One of the cellular pathways which is important for the activation of T-cells is
the LFA-3/CD2 pathway. AMEVIVE(TM) (alafacept) is a recombinantly engineered
protein designed to modulate immune responses by binding to the CD2 receptor.
Biogen is developing AMEVIVE(TM) as a treatment for certain autoimmune diseases.
In 1999, the Company completed a Phase


                                       4

<PAGE>   5

2b clinical study of AMEVIVE(TM) in patients with moderate to severe chronic
plaque psoriasis. Based on positive data from the Phase 2b study, the Company
began Phase 3 studies in North America and Europe. The Company has completed
dosing of the Phase 3 studies, and expects that results of the studies will be
made available in the second quarter of 2001. Psoriasis is a chronic autoimmune
disease that is characterized by inflammation and thickening of the skin. An
estimated 500,000 psoriasis patients in the United States have a severe enough
form of the disease to need systemic therapies. Biogen also has begun initial
work for clinical trials of AMEVIVE(TM) in other indications.


     ANTEGREN(R) (NATALIZUMAB)

     Antegren(R) (natalizumab) is a humanized monoclonal antibody, the first of
a new class of potential therapeutics known as alpha 4 integrin inhibitors.
Antegren(R) is designed to block cell adhesion to blood vessel walls and
subsequent migration of white blood cells into tissue, by binding to the cell
surface receptors known as alpha-4-beta-1 (VLA-4) and alpha-4-beta-7 which are
found on most types of white blood cells. The migration of white blood cells
into tissue is part of the body's normal response during inflammation. This
inflammatory response can be severely damaging or even life threatening when it
is directed against the body's own tissue in autoimmune diseases and may cause
serious collateral injury in chronic immune inflammatory diseases.

     In August of 2000, Biogen entered into a collaborative research,
development and license agreement with Elan Corporation, plc ("Elan") under
which Biogen and Elan are working together to develop, manufacture and
commercialize Antegren(R) worldwide. In January of 2001, Biogen and Elan
announced positive results from two large Phase 2 clinical studies of
Antegren(R) in multiple sclerosis and Crohn's Disease. The companies expect to
initiate Phase 3 clinical studies of Antegren(R) in both of these diseases in
2001.


     ADENOSINE A1 RECEPTOR ANTAGONISTS

     In March 1997, Biogen entered into a research collaboration and license
agreement with CV Therapeutics, Inc. ("CVT") pursuant to which the Company
obtained rights under CVT's patents and know-how to develop and market molecules
that act as highly selective antagonists of the adenosine A1 receptor. The
adenosine A1 receptor is expressed principally in the heart, brain and kidney,
and in the kidney mediates vasoconstriction, renal function and reabsorption of
fluids. Biogen is developing small molecule adenosine A1 receptor antagonists as
a potential treatment for congestive heart failure. Congestive heart failure is
a chronic progressive disease that affects four to five million people in the
United States. Patients with the disease experience both a chronic course as
well as acute episodes of heart failure that usually require hospitalization.
Reduction in kidney function and the formation of edema, or fluid retention, in
lungs and extremities are significant symptoms of chronic heart failure, leading
to increased morbidity, hospitalization and death. In early 2000, Biogen
announced that it had successfully completed a Phase 2 study of a
proof-of-concept adenosine A1 receptor antagonist molecule in patients with
moderate to severe congestive heat failure. Since then, the Company has been
working on the development of a possible commercial compound and anticipates
commencing a Phase 1 study with the commercial compound in 2001 in both oral and
intravenous forms.


                                       5
<PAGE>   6

     LT-BETA RECEPTOR

     The lymphotoxin-beta receptor ("LT-Beta Receptor") pathway is involved in
controlling the maintenance of proper immune interactions and the correct
positioning of key cell types in the immune system. Both elements are crucial
for the immune system to function properly. The LT-Beta Receptor pathway serves
as a novel access point to modulate autoimmune disease. Biogen is developing its
LT-Beta Receptor Ig fusion protein as a potential treatment for certain
autoimmune diseases via various routes of administration and completed dosing in
healthy patients for Phase 1 safety studies in March, 2001.

     GENE THERAPY

     In August 2000, the Company entered into a collaborative research agreement
with Targeted Genetics Corporation, successor in interest to Genovo, Inc.
("Genovo") continuing Biogen's agreement with Genovo for the development of
certain human gene therapy treatments.

     OTHER RESEARCH PROGRAMS

     Biogen's research strategy is to direct its efforts toward finding
therapeutics in four major research focus areas: fibrosis, oncology,
immunomodulation, and neurodegeneration. The Company believes that this focused
research effort along with the leveraging of its traditional strengths in
biologics research, protein chemistry and protein manufacturing will allow the
Company to be in a position to capitalize on the potential of the genomics era.
The Company is exploring the use of functional genomics tools and technology to
find novel therapeutics through its collaborations with Eos Biotechnology, Inc.,
Lexicon Genetics Incorporated, MorphoSys AG, Gene Logic, Inc., Incyte Genomics,
Inc. and Genetica Incorporated. As part of its further research efforts, Biogen
is also exploring the use of growth factors to prevent or treat the degeneration
of organs following damage and investigating new ways to modify immune responses
more specifically in order to treat diseases of the immune system.

     RESEARCH AND DEVELOPMENT COSTS

     For the years ended December 31, 2000, 1999 and 1998, Biogen's research and
development costs were approximately $302.8 million, $221.2 million and $177.2
million, respectively.

     RISKS ASSOCIATED WITH DRUG DEVELOPMENT AND COMMERCIALIZATION

     Certain of the statements set forth above regarding the Company's research
and development programs, such as statements regarding the anticipated
commencement of clinical trials of drugs in development, are forward-looking,
and are based upon the Company's current belief as to the outcome and timing of
such future events. These events are subject to a number of factors and
uncertainties which could cause actual results to differ materially from those
described in the forward-looking statements. Many important factors affect the
Company's ability to successfully develop and commercialize drugs, including the
need to demonstrate the safety and efficacy of drug candidates at each stage of
the clinical trial process, to overcome technical hurdles that may arise, to
meet applicable regulatory standards, to receive required regulatory approvals,
to be capable of producing drug candidates in commercial quantities at
reasonable costs, to obtain and maintain all necessary patents or licenses, to
compete successfully against other products, and to market products
successfully. There can


                                       6

<PAGE>   7


be no assurance that any of the products described in this section or resulting
from Biogen's research and development programs will be successfully developed,
prove to be safe and efficacious at each stage of clinical trials, meet
applicable regulatory standards, be capable of being produced in commercial
quantities at reasonable costs, be successfully marketed or successfully meet
challenges from competitive products.

     For a detailed discussion of the risks associated with the Company's drug
development and commercialization program, see the Company's 2000 Annual Report
to Shareholders --- "Management's Discussion and Analysis of Financial Condition
and Results of Operations --- Outlook," which is incorporated herein by
reference under Item 7 hereof.

PRINCIPAL PRODUCTS BEING MARKETED OR DEVELOPED BY BIOGEN'S  LICENSEES

     ALPHA INTERFERON

     Alpha interferon is a naturally occurring protein produced by normal white
blood cells. Biogen has been granted patents covering the production of alpha
interferon through recombinant DNA techniques. See "Patents and Other
Proprietary Rights." Biogen's worldwide licensee for recombinant alpha
interferon, Schering-Plough Corporation ("Schering-Plough"), first began
commercial sales of its INTRON(R) A brand of alpha interferon in the United
States in 1986 for hairy-cell leukemia. Schering-Plough now sells INTRON(R) A
worldwide for as many as 16 indications. The United States Food and Drug
Administration (the "FDA") has approved INTRON(R) A for the treatment of chronic
hepatitis B and hepatitis C, hairy-cell leukemia, AIDS-related Kaposi's sarcoma,
condylomata acuminata, for injection as an adjuvant treatment to surgery in
patients at high risk for systemic recurrence of malignant melanoma, and for use
in conjunction with anthracycline-containing combination chemotherapy for the
initial treatment of patients with clinically aggressive non-Hodgkin's lymphoma.
Schering-Plough also sells alpha interferon in two other forms, REBETRON(R), a
combination product containing INTRON(R) A and REBETROL(R) (ribavirin, USP
capsules) and PEG-INTRON(R), a pegylated form of alpha interferon recently
approved by the FDA and approved in May 2000 by European Union regulatory
authorities for use in the treatment of hepatitis C.

     Royalties from Schering-Plough on sales of INTRON(R) A accounted for
approximately 10%, 13% and 16% of Biogen's revenues in 2000, 1999 and 1998,
respectively.

     For a discussion of the length of Schering-Plough's royalty obligation to
Biogen on sales of alpha interferon products and a current dispute with
Schering-Plough related to royalty amounts, see "Patents and Other Proprietary
Rights - Recombinant Alpha Interferon."

     HEPATITIS B VACCINES AND DIAGNOSTICS

     Hepatitis B is a blood-borne disease which causes a serious infection of
the liver and substantially increases the risk of liver cancer. More than 250
million people worldwide have chronic hepatitis B virus infections. Biogen holds
several important patents related to hepatitis B antigens produced by genetic
engineering techniques. See "Patents and Other Proprietary Rights - Recombinant
Hepatitis B Antigens." These antigens are used in recombinant hepatitis B
vaccines and in diagnostic test kits used to detect hepatitis B infection.


                                       7

<PAGE>   8

          Hepatitis B Vaccines

     Approximately 100 countries around the world, including the United States,
have added the vaccination against hepatitis B to their routine immunization
programs for all children. The United States Centers for Disease Control and the
American Academy of Pediatrics have also recommended universal immunization of
ten-year-old children and at-risk adolescents. The United States Occupational
Safety and Health Administration has recommended that all persons with an
occupational exposure to blood and other infectious material receive the
hepatitis B vaccine.

     GlaxoSmithKline plc ("SmithKline") and Merck and Co, Inc. ("Merck") are the
two major worldwide marketers of hepatitis B vaccines. Biogen has licensed to
SmithKline exclusive rights under Biogen's hepatitis B patents to market
hepatitis B vaccines in the major countries of the world, excluding Japan.
SmithKline currently pays Biogen royalties based on sales of SmithKline's
vaccine in the United States and in over 15 other countries. In 1990, SmithKline
and Biogen entered into a sublicense arrangement with Merck under which Biogen
currently receives royalties. Biogen has also licensed rights relating to
hepatitis B vaccines under its hepatitis B patents to Merck and The Green Cross
Corporation on a non-exclusive basis in Japan.

          Hepatitis B Diagnostics

     Biogen has licensed its proprietary hepatitis B rights, on an
antigen-by-antigen and nonexclusive basis, to diagnostic kit manufacturers.
Biogen currently has hepatitis B license or license and supply agreements for
diagnostic use with more than 15 companies, including Abbott Laboratories, the
major worldwide marketer of hepatitis B diagnostic kits, Ortho-Clinical
Diagnostics, Organon Teknika B.V. and Roche Diagnostic Systems, Inc.

     For a discussion of the length of the royalty obligation of SmithKline and
Merck on sales of hepatitis B vaccines and the obligation of Biogen's other
licensees on sales of hepatitis B-related diagnostic products, see "Patents and
Other Proprietary Rights - Recombinant Hepatitis B Antigens."

     OTHER PRODUCTS

     In 1996, Biogen granted a sublicense to Pharmacia & Upjohn AB ("Pharmacia &
Upjohn") under certain patent rights to proprietary protein secretion technology
exclusively licensed to Biogen by Harvard University. Under the terms of the
license agreement, Biogen receives ongoing royalties on sales of Pharmacia &
Upjohn's recombinant human growth hormone product, Genotropin(R), in the United
States, Canada and Japan.

     In March 1997, Biogen granted to The Medicines Company ("TMC") exclusive
worldwide rights to develop and market Biogen's bivalirudin product, a direct
thrombin inhibitor now known as ANGIOMAX(TM). Biogen will receive milestone and
royalty payments from TMC if TMC is successful in its efforts to develop and
commercialize the drug. In December 2000, TMC received clearance from the FDA to
market and sell ANGIOMAX(TM) in the United States for use as an anticoagulant in
patients undergoing coronary angioplasty. TMC currently markets ANGIOMAX(TM) in
New Zealand for this use. In Europe, the filing for marketing approval of
ANGIOMAX(TM) is under regulatory review by the European Agency for the
Evaluation of Medicinal Products.


                                       8
<PAGE>   9

     Financial information about foreign operations and export sales is included
in the Company's 2000 Annual Report to Shareholders --- Notes to Consolidated
Financial Statements --- Note 11, incorporated herein by reference under Item 8
hereof.

PATENTS AND OTHER PROPRIETARY RIGHTS

     Biogen has filed numerous patent applications in the United States and
various other countries seeking protection of a number of its processes and
products. Patents have been issued on many of these applications. The Company
has also obtained rights to various patents and patent applications under
licenses with third parties which provide for the payment of royalties by the
Company. The ultimate degree of patent protection that will be afforded to
biotechnology products and processes, including those of Biogen, in the United
States and in other important markets remains uncertain and is dependent upon
the scope of protection decided upon by the patent offices, courts and lawmakers
in these countries. There is no certainty that Biogen's existing patents or
others, if obtained, will afford substantial protection or commercial benefit to
Biogen. Similarly, there is no assurance that the Company's pending patent
applications or patent applications licensed from third parties will ultimately
be granted as patents or that those patents that have been issued or are issued
in the future will prevail if they are challenged in court. There has been, and
Biogen expects that there may continue to be, significant litigation in the
industry regarding patents and other intellectual property rights. Intellectual
property litigation could therefore create uncertainty and consume substantial
resources.

     RECOMBINANT ALPHA INTERFERON

     Biogen has obtained approximately 67 patents in countries around the world,
including the United States and numerous other countries, covering the
production of recombinant alpha interferons. Biogen has granted an exclusive
worldwide license to Schering-Plough under Biogen's alpha interferon patents,
and receives royalties from Schering-Plough on sales of its brands of alpha
interferon, including INTRON(R) A. See "Principal Products Being Marketed or
Developed by Biogen's Licensees - Alpha Interferon". Many of Biogen's alpha
interferon patents have recently expired or will expire during 2001.
Schering-Plough's royalty obligation to Biogen on sales of INTRON(R) A in Japan
and Europe terminated upon expiration of Biogen's alpha interferon patent in
such territories in January 2001, except in France and Italy where Biogen has
obtained supplementary protection certificates extending the coverage in France
until 2003 and in Italy until 2007.

     In December 1996, Schering-Plough filed suit in its own name, as Biogen's
exclusive licensee, against Amgen, Inc. ("Amgen") to enforce Biogen's United
States alpha interferon patent claiming it to be infringed by Amgen's consensus
interferon product known as Infergen(R). In July 1998, the United States
District Court for the District of Delaware construed the claims of the patent
narrowly, which precluded finding Amgen's consensus interferon product to be an
infringing product. Schering-Plough appealed the 1998 ruling and in a ruling by
the Court of Appeals for the Federal Circuit in August 2000, the District
Court's narrow claim construction was affirmed, resulting in a determination
that Amgen's consensus interferon product did not infringe the Biogen patent.

     Schering-Plough has taken the position that as a result of the Court of
Appeal's decision in the Amgen case narrowing the scope of Biogen's United
States alpha interferon patent, the patent no longer covers Schering-Plough's
alpha interferon product, and that, as a result, Schering-Plough no longer has
an obligation to pay royalties under that patent on sales of its alpha
interferon products in the United


                                       9

<PAGE>   10

States. Until January 2001 when Biogen's EU patent expired, Schering-Plough
continued to pay royalties on sales of INTRON(R) A in the United States based on
the manufacture of the product in the EU. The dispute related to the impact of
the Amgen decision on Schering-Plough's royalty obligation does not affect its
obligation to pay royalties on U.S. sales of its alpha interferon products after
July 2002. In consideration of assignment to Schering-Plough by Biogen of a
Biogen patent application claiming recombinant mature human alpha interferon,
Schering-Plough has agreed to pay to Biogen certain sums on sales by
Schering-Plough of alpha interferon products in the United States from the date
when Biogen's existing United States alpha interferon patent expires (i.e., July
2002) until expiration of an alpha interferon patent expected to be issued to
Hoffman-LaRoche Inc. ("Roche") and Genentech, Inc. ("Genentech"). The
Roche/Genentech patent was the subject of a lawsuit brought by Biogen which was
ultimately settled. Schering-Plough entered into an agreement with Roche as part
of the settlement of the matter. Biogen is currently in discussions with
Schering-Plough to work to resolve the twelve to eighteen month royalty issue
and to resolve claims by Biogen related to underpayment of royalties by
Schering- Plough.

     RECOMBINANT HEPATITIS B ANTIGENS

     Biogen has obtained numerous patents in countries around the world,
including in the United States and in European countries, covering the
recombinant production of hepatitis B surface, core and "e" antigens. Biogen has
licensed its recombinant hepatitis B antigen patent rights to manufacturers and
marketers of hepatitis B vaccines and diagnostic test kits, and receives
royalties on sales of the vaccines and test kits by its licensees. See
"Principal Products Being Marketed or Developed by Biogen's Licensees -
Hepatitis B Vaccines and Diagnostics." The obligation of SmithKline and Merck to
pay royalties on sales of hepatitis B vaccines and the obligation of Biogen's
other licensees under its hepatitis B patents to pay royalties on sales of
diagnostic products will terminate upon expiration of Biogen's hepatitis B
patents in each licensed country. Biogen's existing United States hepatitis B
patent will expire in 2004. Biogen's European hepatitis B patents expired at the
end of 1999, except in those countries in which Biogen has obtained
supplementary protection certificates. To date, Biogen has received
supplementary protection certificates in Austria, Belgium, France, Great
Britain, Ireland, Italy, Luxembourg, The Netherlands, Sweden and Switzerland,
and has a number of granted or pending registrations of the Great Britain
supplementary protection certificates in various British Territories. The
additional coverage afforded by the supplementary protection certificates, or
related registrations, ranges from two to eight years.

     RECOMBINANT BETA INTERFERON

     In 1997, the Technical Board of Appeal of the European Patent Office
revoked Biogen's European patent covering the production of recombinant beta
interferon. Biogen appealed the decision, but a recent decision in another case
denied that such a right of appeal exists. Consequently, Biogen's appeal stands
dismissed and the patent stands revoked. Biogen had sued InterPharm Laboratories
Ltd. ("InterPharm"), an affiliate of Ares Serono, S.A. ("Serono"), and related
defendants under a related Israeli patent claiming that the manufacture by
InterPharm of Serono's Rebif(R) (Interferon Beta-1a) infringes Biogen's Israeli
patent. In October 2000, Biogen reached a settlement with the defendants and the
case was dismissed in Israel. In Germany, a patent with similar claims was the
subject of a nullity proceeding instituted by Schering AG in the German Federal
Patent Court. In March 1998, the German Federal Patent Court upheld the German
patent but with substantially narrower claims. Biogen has appealed the decision
but does not anticipate any decision until sometime during 2002.


                                       10

<PAGE>   11

     Other parties have pending patent applications or issued patents in the
United States, Europe and other countries with claims to key intermediates in
the production of beta interferon (the "Taniguchi patents") and to beta
interferon itself (the "Roche patents"). Biogen has obtained non-exclusive
rights in various countries of the world, including the United States, Japan and
most European countries, to manufacture, use and sell AVONEX(R) under the
Taniguchi patents and has obtained worldwide, non-exclusive rights under the
Roche patents.

     On July 3, 1996, Berlex Laboratories, Inc. ("Berlex") filed suit against
Biogen in the United States District Court for the District of New Jersey
alleging infringement by Biogen of Berlex's "McCormick" patent (U.S. Patent No.
5,376,567) in the United States in the production AVONEX(R). In November 1996,
Berlex's New Jersey action was transferred to the United States District Court
in Massachusetts and consolidated for pre-trial purposes with a related
declaratory judgment action previously filed by Biogen. On August 18, 1998,
Berlex filed a second suit against Biogen alleging infringement by Biogen of a
patent which was issued to Berlex in August 1998 and which is related to the
McCormick patent (U.S. Patent No. 5,795,779). On September 23, 1998, the cases
were consolidated for pre-trial and trial purposes. Berlex sought a judgment
granting it damages, a trebling of any damages awarded and a permanent
injunction restraining Biogen from the alleged infringement. A hearing on the
parties' summary judgment motions in the case was completed in March 2000. In
September 2000, the District Court rendered final judgment in favor of Biogen
and against Berlex determining that Biogen's production of AVONEX(R) did not
infringe any of the claims of the Berlex patents. Berlex has appealed this
decision to the Court of Appeals for the Federal Circuit, and the parties are in
the process of briefing the matter for oral argument before the court. For a
further discussion, see Item 3 hereof - Legal Proceedings, and the Company's
2000 Annual Report to Shareholders --- Notes to Consolidated Financial
Statements --- Note 9, incorporated herein by reference under Item 8 hereof.

     In 1995, the Company filed an opposition with the Opposition Division of
the European Patent Office opposing a European patent (the "Rentschler I
Patent") issued to Dr. Rentschler Biotechnologie GmbH ("Rentschler") relating to
compositions of matter of beta interferon. In 1997, the European Patent Office
issued a decision revoking the Rentschler I Patent. Rentschler appealed that
decision. An oral hearing on the appeal took place in December 2000. At the oral
hearing in order to gain reinstatment of the patent, Rentschler narrowed the
patent claims so as to claim only a specific cell line. Biogen does not use the
specific cell line now claimed. On October 13, 1998, the Company filed another
opposition with the Opposition Division of the European Patent Office to oppose
a second European patent issued to Rentschler (the "Rentschler II Patent") with
certain claims regarding compositions of matter of beta interferon with specific
regard to the structure of the glycosylated molecule. The parties are in the
process of filing further evidence and arguments with the European Oppositon
Division. A decision on the Rentschler II Patent has not been issued to date.
For a more detailed discussion, see the Company's 2000 Annual Report to
Shareholders --- "Management's Discussion and Analysis of Financial Condition
and Results of Operations - Legal Matters" incorporated herein by reference
under Item 7 hereof.


     OTHER PATENTS

     In March 1995, Biogen filed suit in the U.S. District Court for the
District of Massachusetts seeking to enjoin Amgen from manufacturing and selling
its Neupogen(R) human granulocyte colony stimulating factor in the United States
and asking for damages for infringing activities. Biogen believes


                                       11

<PAGE>   12

that to make Neupogen(R) Amgen uses technology claimed in certain of Biogen's
gene expression patents. In 1998, the court interpreted the broad vector claim
of one of the Biogen patents in such a way as to preclude Amgen's literal
infringement of the claim. Amgen thereafter filed a motion for summary judgment
based on the court's 1998 claim construction as well as a motion for a
declaration of noninfringement under the doctrine of equivalents. The matter was
argued to the court in November 1999 and the Court issued a decision in
September 2000 allowing Amgen's motions for a declaration of noninfringement.
Amgen made a motion asking the court to apply its decisions with respect to
noninfringement to the remaining Biogen patents in suit and arguing that Biogen
may not assert infringement under the doctrine of equivalents as a result of
arguments Biogen made in obtaining the patent. On March 1, 2001, the court ruled
in favor of Amgen with respect to one of the patents in the suit and ordered a
hearing on Amgen's arguments with respect to the remaining patents in the suit.
Biogen intends to seek reconsideration of the court's ruling.

     In July 1997, Biogen filed suit in the U.S. District Court for the District
of Massachusetts to enjoin Amgen from manufacturing and selling its Infergen(R)
consensus interferon in the United States and asking for damages for infringing
activities. Biogen believes that to make Infergen(R) Amgen uses technology
claimed in certain of Biogen's gene expression patents. Biogen's request to have
the case consolidated with the Neupogen(R) suit was denied by the court. The
case has been stayed pending resolution of the Biogen suit pertaining to
Neupogen(R). In the court's March 1, 2001 decision in the Neupogen(R) suit, the
court ordered a hearing on Amgen's contention that the court's ruling in the
Neupogen(R) suit compels dismissal of the Infergen(R) suit. In 1994, Biogen
granted Eli Lilly and Company ("Lilly") a non-exclusive license under the same
patents for gene expression involved in the Amgen case. Under the license,
Biogen has received royalties from Lilly since 1994 on products which use the
patented vectors and methods. Based on the court's claims construction decision
in the Amgen case, Lilly recently has notified Biogen that Lilly believes that
it no longer owes royalties to Biogen under the agreement on any of its
products. Biogen's European patent relating to gene expression was opposed by
Biotechnology General Corp. in December 1993. A hearing was held by the
Opposition Division of the European Patent Office in March 1996. In March 1997,
the Opposition Division decided to revoke Biogen's patent. Biogen appealed the
decision and a hearing is scheduled on Biogen's appeal in June 2001. Biogen
successfully defended a similar opposition against issuance of a counterpart
patent in Japan.

     THIRD-PARTY PATENTS

     Biogen is aware that others, including various universities and companies
working in the biotechnology field, have also filed patent applications and have
been granted patents in the United States and in other countries claiming
subject matter potentially useful or necessary to Biogen's business. Some of
those patents and patent applications claim only specific products or methods of
making such products, while others claim more general processes or techniques
useful or now used in the biotechnology industry. For example, Genentech has
been granted patents and is prosecuting other patent applications in the United
States and certain other countries which it may allege are currently used by
Biogen and the rest of the biotechnology industry to produce recombinant
proteins in microbial hosts. Genentech has offered to Biogen and others in the
industry non-exclusive licenses under those patents and patent applications for
various proteins and in various fields of use, but not for others. The ultimate
scope and validity of Genentech's patents, of other existing patents, or of
patents which may be granted to third parties in the future, and the extent to
which Biogen may wish or be required to acquire rights


                                       12

<PAGE>   13


under such patents and the availability and cost of acquiring such rights,
currently cannot be determined by Biogen.

     TRADE SECRETS AND CONFIDENTIAL KNOW-HOW

     Trade secrets and confidential know-how are important to Biogen's
scientific and commercial success. Although Biogen seeks to protect its
proprietary information by generally requiring its employees, consultants,
advisors and corporate partners to sign confidentiality agreements, there can be
no assurance that third parties will not either independently develop the same
or similar information or obtain access to Biogen's proprietary information.

COMPETITION AND MARKETING

     IN GENERAL

     Competition in the biotechnology and pharmaceutical industries is intense
and comes from many and varied sources. Biogen does not believe that it or any
of the other industry leaders can be considered dominant in view of the rapid
technological change in the industry. Biogen experiences significant competition
from specialized biotechnology firms in the United States, Europe and elsewhere
and from many large pharmaceutical, chemical and other companies. Certain of
these companies have substantially greater financial, marketing, research and
development and human resources than Biogen. Most pharmaceutical companies have
considerable experience in undertaking clinical trials and in obtaining
regulatory approval to market pharmaceutical products.

     Biogen believes that competition and leadership in the industry will be
based on managerial and technological superiority and establishing proprietary
positions through research and development. Leadership in the industry may also
be influenced significantly by patents and other forms of protection of
proprietary information. See "Patents and Other Proprietary Rights". A key
aspect of such competition is recruiting and retaining qualified scientists and
technicians. Biogen believes that it has been successful in attracting skilled
and experienced scientific personnel. The achievement of a leadership position
depends largely upon Biogen's continued ability to attract and retain skilled
and experienced personnel, its ability to identify and exploit commercially the
products resulting from research and the availability of adequate financial
resources to fund facilities, equipment, personnel, clinical testing,
manufacturing and marketing.

     Many of Biogen's competitors are working to develop products similar to
those under development by Biogen. The timing of the entry of a new
pharmaceutical product into the market can be an important factor in determining
the product's eventual success and profitability. Early entry may have important
advantages in gaining product acceptance and market share. Moreover, for certain
diseases with limited patient populations, the FDA is prevented under the Orphan
Drug Act, for a period of seven years, from approving more than one application
for the "same" product for a single orphan drug designation, unless a later
product is considered clinically superior. The EU and other jurisdictions have
or are considering similar laws. Accordingly, the relative speed with which
Biogen can develop products, complete the testing and approval process and
supply commercial quantities of the product to the market will have an important
impact on Biogen's competitive position. In addition, competition among products
approved for sale may be based, among other things, on patent position, product
efficacy, safety, reliability, availability and price.


                                       13

<PAGE>   14

     AVONEX(R) (INTERFERON BETA - 1a)

     As a treatment for multiple sclerosis, AVONEX(R) competes with interferon
beta-1b which is sold in the United States under the brand name Betaseron(R) by
Berlex Laboratories, a United States affiliate of Schering AG, and is sold in
Europe under the brand name Betaferon(R) by Schering AG. AVONEX(R) also faces
competition from Copaxone(R) glatiramer acetate (also known as copolymer-1). In
the United States, Copaxone(R) is marketed by a partnership between Teva
Pharmaceutical Industries, Ltd. and Hoechst Marion Roussel, Inc. In most
countries outside of the United States, AVONEX(R) also competes with Rebif(R), a
recombinant interferon beta-1a product sold by Serono. In response to an
application from Serono for approval of Rebif(R) in the United States for
relapsing multiple sclerosis, the FDA, in March 1999, upheld its earlier ruling
that, based on the data from existing clinical trials, Serono cannot market
Rebif(R) in the United States for relapsing multiple sclerosis while the orphan
drug status afforded to AVONEX(R) and Betaseron(R) for that indication is still
in effect. AVONEX(R)'s orphan drug status for relapsing forms of the disease
expires in 2003. The ruling by the FDA prompted Serono in 2000 to initiate a
12-month head-to-head study of Rebif(R) and AVONEX(R) to determine if Serono can
show whether Rebif(R) is clinically superior to AVONEX(R). If positive, Serono
will most likely use the results of this study in its attempts to overcome the
orphan drug status of AVONEX(R) and to get Rebif(R) approved in the United
States before 2003. Biogen expects Serono to release the results of the study in
the third quarter of 2001. AVONEX(R) also competes in the United States with
Novantrone(R) (mitoxantrone for injection) which is produced and marketed by
Immunex Corporation, a majority-owned subsidiary of American Home Products
Corporation. Novantrone(R) is approved for use in patients with clinically
worsening forms of relapsing-remitting and secondary progressive multiple
sclerosis.

     A number of other companies are working to develop products to treat
multiple sclerosis which may in the future compete with AVONEX(R), the worldwide
market leader among multiple sclerosis therapies. AVONEX(R) may also in the
future face competition from off-label uses of drugs approved for other
indications. Biogen believes that competition among treatments for multiple
sclerosis will be based on product performance, service and price.

REGULATION

     Biogen's current and contemplated activities and the products and processes
that will result from such activities are, and will be, subject to substantial
government regulation.

     Before new pharmaceutical products may be sold in the United States and
other countries, clinical trials of the products must be conducted and the
results submitted to appropriate regulatory agencies for approval. These
clinical trial programs generally involve a three-phase process. Typically, in
Phase 1, trials are conducted in volunteers or patients to determine the early
side effect profile and, perhaps, the pattern of drug distribution and
metabolism. In Phase 2, trials are conducted in groups of patients with a
specific disease in order to determine appropriate dosages, expand evidence of
the safety profile and, perhaps, determine preliminary efficacy. In Phase 3,
large scale, comparative trials are conducted on patients with a target disease
in order to generate enough data to provide the statistical proof of efficacy
and safety required by national regulatory agencies. The receipt of regulatory
approvals often takes a number of years, involving the expenditure of
substantial resources and depends on a number of factors, including the severity
of the disease in question, the availability of alternative


                                       14

<PAGE>   15

treatments and the risks and benefits demonstrated in clinical trials. On
occasion, regulatory authorities may require larger or additional studies,
leading to unanticipated delay or expense.

     In connection with the commercialization of products resulting from
Biogen's research and development projects, it is necessary, in a number of
countries, to comply with certain regulations relating to the manufacturing and
marketing of such products and to the products themselves. For example, the
commercial manufacturing, marketing and exporting of pharmaceutical products
require the approval of the FDA in the United States and of comparable agencies
in other countries. The FDA has established mandatory procedures and safety
standards which apply to the manufacture, clinical testing and marketing of
pharmaceutical products in the United States. The regulatory requirements and
approval processes for new products in the EU operate under similar principles
as those applied in the United States. The process of seeking and obtaining
approval of the FDA or regulatory authorities in the EU or other regulatory
authorities worldwide for a new product and licensing of the facilities in which
the product is produced takes a number of years and involves the expenditure of
substantial resources. In addition, the regulatory approval processes for
products in the United States, the countries of the EU and other countries
around the world are undergoing or may undergo changes. Biogen cannot determine
what effect any changes in regulatory approval processes may have on its
business.

     In the United States, the federal government regularly considers reforming
health care coverage and costs. Resulting legislation or regulatory actions may
have a significant effect on the Company's business. Biogen's ability to
successfully commercialize human pharmaceutical products also may depend in part
on the extent to which reimbursement for the costs of such products and related
treatments will be available worldwide from government health administration
authorities, private health insurers and other organizations. Currently,
substantial uncertainty exists as to the reimbursement status of newly approved
health care products by third-party payors.

     Biogen conducts relevant research in compliance with the current United
States National Institutes of Health Guidelines for Research Involving
Recombinant DNA Molecules (the "NIH Guidelines") and all other applicable
federal and state regulations. By local ordinance, Biogen is required, among
other things, to comply with the NIH Guidelines in relation to its facilities in
Cambridge, Massachusetts, and is required to operate pursuant to certain
permits.

     Various laws, regulations and recommendations relating to safe working
conditions, laboratory practices, the experimental use of animals, and the
purchase, storage, movement, import and export and use and disposal of hazardous
or potentially hazardous substances, including radioactive compounds and
infectious disease agents, used in connection with Biogen's research work are or
may be applicable to its activities. The extent of government regulation which
might result from future legislation or administrative action cannot accurately
be predicted. Certain agreements entered into by Biogen involving exclusive
license rights may be subject to national or supranational antitrust regulatory
control, the effect of which also cannot be predicted.

EMPLOYEES

     At December 31, 2000, Biogen employed 1,475 full-time employees worldwide,
of whom 1,217 were located in the United States. Of the 1,475 employees, 424
were engaged in, or directly supported, research, including medical research,
454 were involved in, or directly supported, manufacturing, product and process
development, and quality assurance/quality control, and 291 were involved in
sales


                                       15

<PAGE>   16


and marketing. In addition, Biogen maintains consulting arrangements with a
number of scientists at various universities and other research institutions in
Europe and the United States, including the ten outside members of its
Scientific Board.

ITEM 2 - PROPERTIES

     Biogen's principal executive offices and a majority of its administrative,
manufacturing and research and development facilities are located in Cambridge,
Massachusetts. The Company owns a 150,000 square foot building in Cambridge that
houses laboratories and office space. The Company also leases a total of
approximately 482,987 square feet of additional office, manufacturing, and
research and development space in all or part of six other buildings in
Cambridge, consisting of: a 67,362 square foot building housing manufacturing
facilities, laboratories and office space; a building with 65,792 square feet of
space containing laboratories, purification and aseptic bottling facilities, and
office space; a multi-tenant building where the Company leases approximately
156,069 square feet of office space; a 17,000 square foot building housing
office space and distribution facilities; a 24,100 square foot warehouse
facility; and a 152,164 square foot office building. The lease expiration dates
for the leased sites range from 2002 to 2015. In February 2001, Biogen completed
construction of a new 224,000 square foot facility in Cambridge primarily to
house research and development and process development operations. The Company
also has development options for additional property in Cambridge.

     In addition to its Cambridge facilities, the Company has a 100,000 square
foot biologics manufacturing facility in Research Triangle Park, North Carolina.
The Company uses the Research Triangle Park facility as a site for the
manufacture of AVONEX(R). In 1999, the Company commenced construction of a
250,000 square foot addition to the Research Triangle Park facility to add large
scale cell culture manufacturing capacity which it expects to complete in the
first half of 2001. In addition, in 2000, the Company commenced construction at
the Research Triangle Park location of a 150,000 square foot office and
laboratory facility.

     Biogen financed construction of the buildings which it owns in Cambridge,
Massachusetts and Research Triangle Park, North Carolina with term loans. The
loans are secured by the buildings. See the Company's 2000 Annual Report to
Shareholders --- "Management's Discussion and Analysis of Financial Condition
and Results of Operations" incorporated herein by reference under Item 7 hereof.

     The Company's European headquarters consists of 4,150 square meters of
office space in a multi-tenant building in Nanterre, France and the company is
currently negotiating for an additional 1,500 square meters. The lease for the
Nanterre space terminates in 2008 with Biogen having the right to terminate the
lease earlier under specified circumstances. The Company also leases 2,250
square meters of office and manufacturing space in The Netherlands, 1,400 square
meters of office space in Germany, of which 450 square meters is subleased to a
third party through the second quarter of 2001, 800 square meters of office
space in England, 450 square meters of office space in Denmark and small offices
in Austria, Canada, Finland, Norway and Sweden.

     The Company believes that its production plants in Cambridge, Massachusetts
and Research Triangle Park, North Carolina and existing outside sources will
allow it to meet, in the near term, its production needs for products in
clinical trials and AVONEX(R). Biogen believes that its existing facilities are
in compliance with applicable regulatory standards. The Company expects that
additional


                                       16

<PAGE>   17

facilities and outside sources will be required to meet the Company's future
research, development and commercial production needs.

ITEM 3 - LEGAL PROCEEDINGS

     On July 3, 1996, Berlex Laboratories, Inc. ("Berlex") filed suit against
Biogen in the United States District Court for the District of New Jersey
alleging infringement by Biogen of Berlex's "McCormick" patent (U.S. Patent No.
5,376,567) in the United States in the production of AVONEX(R). In November
1996, Berlex's New Jersey action was transferred to the United States District
Court in Massachusetts and consolidated for pre-trial purposes with a related
declaratory judgment action previously filed by Biogen. On August 18, 1998,
Berlex filed a second suit against Biogen alleging infringement by Biogen of a
patent which was issued to Berlex in August 1998 and which is related to the
McCormick patent (U.S. Patent No. 5,795,779). On September 23, 1998, the cases
were consolidated for pre-trial and trial purposes. Berlex sought a judgment
granting it damages, a trebling of any damages awarded and a permanent
injunction restraining Biogen from the alleged infringement. A hearing on the
parties' summary judgment motions in the case was completed in March 2000. In
September 2000, the District Court rendered final judgment in favor of Biogen
and against Berlex determining that Biogen's production of AVONEX(R) did not
infringe any of the claims of the Berlex patents. Berlex has appealed this
decision with the Court of Appeals for the Federal Circuit and the parties are
in the process of briefing the appeal for oral argument. An unfavorable ruling
on appeal would result in the case being remanded to the District Court for
trial. If Berlex were to be successful in its appeal and the case were to be
remanded, an unfavorable ruling in the remanded case could have a material
adverse effect on the Company's results of operations and financial position.
The Company believes that the decision of the District Court that Biogen does
not infringe the Berlex patents is sound, but the ultimate outcome of the appeal
is not currently determinable. As a result, an estimate of any potential loss or
range of loss cannot be made at this time.

     For a description of legal proceedings relating to certain patent rights,
see Item 1 hereof, "Business - Patents and Other Proprietary Rights."


ITEM 4 - SUBMISSION OF MATTERS TO A VOTE OF SECURITY HOLDERS

     Not Applicable


                                       17

<PAGE>   18


     EXECUTIVE OFFICERS OF THE REGISTRANT

     The following is a list of each executive officer of the Company, their
respective age as of December 31, 2000 and their principal positions with the
Company. Officers are elected and may be removed by the Board of Directors.


<TABLE>
<CAPTION>
Name                           Age          Positions
----                           ---          ---------

<S>                            <C>          <C>
James L. Vincent...........    61           Chairman of the Board of Directors

James C. Mullen............    42           President, Chief Executive Officer and Director

Burt A. Adelman............    48           Vice President - Medical Research

Cornelis "Kees" Been.......    42           Vice President - Business and Market Development

Thomas J. Bucknum..........    54           Vice President - General Counsel, Secretary and Clerk

Frank A. Burke, Jr.........    57           Vice President - Human Resources

Nadine D. Cohen............    51           Vice President - Regulatory Affairs

Michael Gilman ............    45           Vice President - Research

Sylvie L. Gregoire.........    39           Vice President - Manufacturing

Robert A. Hamm.............    49           Vice President - Sales and Marketing

Peter N. Kellogg ..........    44           Vice President - Finance and Chief Financial Officer

Mark W. Leuchtenberger.....    44           Vice President - International

Toshio Nakata..............    57           President - Biogen Japan, Ltd.,
                                            Vice President - Japan, Asia and Oceana

John W. Palmer.............    49           Vice President - Program Management

David D. Pendergast........    52           Vice President - Product Development and Quality Assurance

Patrick J. Purcell.........    40           Vice President - Information Systems and Chief Information
                                            Officer
</TABLE>

The background of these officers is as follows:

     James L. Vincent has been Chairman of the Board of Directors since October
1985. Mr. Vincent served as Chief Executive Officer from December 1998 to June
2000. He previously served as Chief

                                       18

<PAGE>   19

Executive Officer of the Company from October 1985 until February 1997. He
served in the additional capacities of Chief Operating Officer and President
from April 1988 until February 1994. Before joining Biogen, Mr. Vincent served
as Group Vice President, Allied Corporation and as President, Allied Health &
Scientific Products Company, a subsidiary of Allied Corporation. Before joining
Allied Corporation, Mr. Vincent was with Abbott Laboratories, Inc. where he
served in various capacities, including Executive Vice President, Chief
Operating Officer and Director of the parent corporation.

     James C. Mullen was appointed President and Chief Executive Officer in June
2000 after serving as President and Chief Operating Officer since January 1999.
He was appointed as a Director of the Company in April 1999. Mr. Mullen
previously served as Vice President - International from August 1996 until
January 1999, and Vice President - Operations from December 1991 until August
1996 and served as Senior Director - Operations from February 1991 to December
1991. Mr. Mullen joined the Company in 1989. Before coming to Biogen, Mr. Mullen
held various positions of responsibility from 1984 through 1988 at
SmithKline-Beckman Corporation (now GlaxoSmithKline plc), including Director,
Engineering, SmithKline and French Laboratories, Worldwide.

     Burt A. Adelman, M.D. was appointed Vice President - Medical Research in
January 1999 after serving as Vice President - Development Operations since
August 1996. Dr. Adelman served as Vice President - Regulatory Affairs of the
Company from May 1995 until August 1996. From 1991 until May 1995, Dr. Adelman
was Director of Medical Research at Biogen. Dr. Adelman has served as Lecturer
of Medicine at Harvard Medical School and Brigham and Women's Hospital since
1992.

     Cornelis "Kees" Been was appointed Vice President - Business and Market
Development in August 1999. Prior to joining the Company, Mr. Been held a
variety of management positions from 1996 until April 1999 with Monsanto Life
Sciences, most recently as Vice President - Global Strategy. From 1988 through
1995, Mr. Been worked at Gemini Consulting, where his positions included Vice
President, responsible for building Gemini's pharmaceuticals practice. Mr. Been
began his career in 1983 as a Trade and Licensing Manager with Biogen, based in
Geneva, Switzerland.

     Thomas J. Bucknum was appointed Vice President - General Counsel, Secretary
and Clerk in July 1999, after serving as the Company's Chief Corporate Counsel
since 1996. Prior to joining the Company, Mr. Bucknum was Senior Vice President
and General Counsel of DuPont Merck Pharmaceutical Company from 1990 to 1995
with responsibility for legal, government and public affairs matters. Prior to
joining DuPont Merck, Mr. Bucknum held a number of domestic and international
positions with E.I. DuPont de Nemours & Company, Inc in the legal, marketing and
regulatory affairs departments.

     Frank A. Burke, Jr., was appointed Vice President - Human Resources in May
1986 after serving for 12 years in various human resource management positions
at Allied-Signal, Inc., including Director of Compensation and Employee Benefits
of the Engineered Materials Sector.

     Nadine D. Cohen, Ph.D., was appointed Vice President - Regulatory Affairs
in October 2000 after serving as Director of Regulatory Affairs since September
1999. Dr. Cohen joined Biogen in 1999 from the Massachusetts Biologics
Laboratories, where she was Senior Director of Quality Control and Technical
Services. Prior to that, she was Vice President of Regulatory Affairs and
Quality at Alpha Beta Technology, Inc.

                                       19

<PAGE>   20

     Michael Gilman, Ph.D., was appointed Vice President - Research in April
2000. Dr. Gilman joined Biogen in 1999 as Director, Molecular Biology from ARIAD
Pharmaceuticals, Inc., where he served as Executive Vice President and Chief
Scientific Officer. From 1986-1994, Dr. Gilman worked at Cold Spring Harbor
Laboratory.

     Sylvie L. Gregoire, Pharm.D., was appointed Vice President - Manufacturing
in October 2000 after serving as Vice President - Regulatory Affairs from
January 1999 to October 2000. From July 1998 to January 1999, Dr. Gregoire was
the Program Executive for the Company's LT-Beta Receptor program and from 1995
until July 1998, served as Director, European Regulatory Affairs of the Company.
Prior to joining Biogen, Dr. Gregoire was Associate Director of European
Regulatory Affairs for Merck Sharp and Dohme (Europe) Inc. from 1991 until the
end of 1994.

     Robert A. Hamm was appointed Vice President - Sales and Marketing in
October 2000 after serving as Vice President - Manufacturing from June 1999 to
October 2000. Mr. Hamm served as Director, Northern Europe and Distributors of
the Company from November 1996 until June 1999 and as the Company's Associate
Director, Logistics from April 1994 until November 1996. From 1987 until April
1994, Mr. Hamm held a variety of management positions at Syntex Laboratories
Corporation, including Director of Operations and New Product Planning, and
Manager of Materials, Logistics and Contract Manufacturing.

     Peter N. Kellogg was appointed Vice President - Finance and Chief Financial
Officer in July 2000. Mr. Kellogg joined Biogen from PepsiCo where from 1987 to
2000, he served in a variety of senior financial, international and general
management positions, including Senior Vice President, PepsiCo. E-Commerce from
March to July 2000. From March 1998 to March 2000, he served as Senior Vice
President and Chief Financial Officer, Frito-Lay International; from November
1996 to March 1998 as Vice President and Chief Financial Officer, Frito-Lay
Latin America; from March 1994 to November 1996 as Vice President and Chief
Financial Officer, Central/Eastern Europe and Russia, Pepsi-Cola International;
and from February 1993 through March 1994 as Vice President and General
Manager-Pepsi, South Franchise Business Unit. Prior to joining PepsiCo, Mr.
Kellogg was a senior consultant with Booz Allen & Hamilton and Arthur Andersen &
Co.

     Mark W. Leuchtenberger was appointed Vice President - International in
January 1999 after serving as Vice President - Sales, Marketing and Business
Development since January 1998. Mr. Leuchtenberger was the Company's Vice
President - Marketing and Sales from October 1996 until January 1998, Director
of Distributor Operations, Europe from September 1996 until October 1996,
Director of Marketing and the Program Executive for AVONEX(R) from 1993 until
September 1996, a Product Manager from 1992 to 1993, and a Market Development
Manager from 1990 to 1992. Prior to joining Biogen, Mr. Leuchtenberger worked
for the consulting firm of Bain & Company from 1987 to 1990.

     Toshio Nakata was appointed President - Biogen Japan, Ltd. and Vice
President - Japan, Asia and Oceana in October 2000. Mr. Nakata joined Biogen
from Mitsui & Co., Ltd. where he served as: Associate Director and General
Manager of the Global Environment Department, Corporate Planning Division, from
January 2000 to October 2000; General Manager of New Business and Technology
Development Department and Global Environment Department, Corporate Planning
Division, from April 1997 to January 2000; and General Manager of Technology
Planning and Development

                                       20

<PAGE>   21

Department and Global Environment Department, Corporate Planning Division, from
May 1994 to April 1997.

     John W. Palmer was appointed as Vice President - Program Management in May
2000, after serving as Program Executive for Biogen's Adensoine A1 antagonist
program since 1997. From 1993 until 1997, Mr. Palmer was the Company's Director
of Operations, and from 1989 until 1993 served as Director of Marketing and
Business Development. Prior to joining Biogen, Mr. Palmer was a Marketing and
Business Development Director at General Foods Corporation. He was also
previously Founder, President and Chief Operating Officer of Caribbean Emergency
Medical Air Transport, Inc., a novel air ambulance service operating throughout
the Caribbean market, and a managing consultant with Strategic Planning
Associates in Washington, D.C.

     David D. Pendergast, Ph.D. was appointed Vice President - Product
Development and Quality Assurance in January 1998 after serving as Vice
President - Quality Assurance and Quality Control of the Company since April
1996. Dr. Pendergast joined Biogen from Fisons Pharmaceuticals, Manchester U.K.
where he served as Director, Quality Assurance/Quality Control of Fisons PLC
from 1992 to 1996. Prior to joining Fisons, Dr. Pendergast served, over a
twenty-year period, in various capacities at The Upjohn Company, including Vice
President - Quality Assurance from 1989 to 1992.

     Patrick J. Purcell was appointed Vice President - Information Systems and
Chief Information Officer in February 2000. Mr. Purcell joins Biogen from Sprint
PCS, where he worked from 1992 to 2000 in a variety of finance and information
technology management capacities, most recently serving as Vice President, IT
Planning, Resource & Performance Management. Prior to joining Sprint, Mr.
Purcell worked at Deloitte & Touche Management Consulting from 1988 to 1992, as
a consulting manager with primary focus on business and financial planning,
financing and operations analysis, and change management in a variety of
industries.


PART II

ITEM 5 - MARKET FOR REGISTRANT'S COMMON EQUITY AND RELATED STOCKHOLDER MATTERS

     The section entitled "Market for Securities" in the Company's 2000 Annual
Report to Shareholders is hereby incorporated herein by reference.

ITEM 6 - SELECTED FINANCIAL DATA

     The section entitled "Selected Financial Data" in the Company's 2000 Annual
Report to Shareholders is hereby incorporated herein by reference.

ITEM 7 - MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND RESULTS
         OF OPERATIONS

     The section entitled "Management's Discussion and Analysis of Financial
Condition and Results of Operations" in the Company's 2000 Annual Report to
Shareholders is hereby incorporated herein by reference.

ITEM 7A - QUANTITATIVE AND QUALITATIVE DISCLOSURES ABOUT MARKET RISK


                                       21

<PAGE>   22

     The section entitled "Management's Discussion and Analysis of Financial
Condition and Results of Operations - Outlook - Market Risk" in the Company's
2000 Annual Report to Shareholders is hereby incorporated herein by reference.

ITEM 8 - FINANCIAL STATEMENTS AND SUPPLEMENTARY DATA

     The sections entitled "Consolidated Statements of Income," "Consolidated
Balance Sheets," "Consolidated Statements of Cash Flows," "Consolidated
Statements of Shareholders' Equity," "Notes to Consolidated Financial
Statements" and "Report of Independent Accountants" in the Company's 2000 Annual
Report to Shareholders are hereby incorporated herein by reference.

ITEM 9 - CHANGES IN AND DISAGREEMENTS WITH ACCOUNTANTS ON ACCOUNTING AND
FINANCIAL DISCLOSURE

     Not Applicable

PART III

ITEM 10 - DIRECTORS AND EXECUTIVE OFFICERS OF THE REGISTRANT

     The sections entitled "Election of Directors" and "Section 16(a) Beneficial
Ownership Reporting Compliance" in the Company's definitive proxy statement for
its 2001 Annual Meeting of Stockholders, which the Company intends to file with
the Commission no later than April 30, 2001, are hereby incorporated herein by
reference.

     Information concerning the Company's Executive Officers is set forth in
Item 4 of Part I of this Annual Report on Form 10-K.

ITEM 11 - EXECUTIVE COMPENSATION

     The sections entitled "Election of Directors" and "Executive Compensation"
in the Company's definitive proxy statement for its 2001 Annual Meeting of
Stockholders, which the Company intends to file with the Commission no later
than April 30, 2001, are hereby incorporated herein by reference.

ITEM 12 - SECURITY OWNERSHIP OF CERTAIN BENEFICIAL OWNERS AND MANAGEMENT

     The section entitled "Share Ownership" in the Company's definitive proxy
statement for its 2001 Annual Meeting of Stockholders, which the Company intends
to file with the Commission no later than April 30, 2001, is hereby incorporated
herein by reference.

ITEM 13 - CERTAIN RELATIONSHIPS AND RELATED TRANSACTIONS

     The section entitled "Executive Compensation - Employment Arrangements with
the Company and Certain Transactions" in the Company's definitive proxy
statement for its 2001 Annual Meeting of Stockholders, which the Company intends
to file with the Commission no later than April 30, 2001, is hereby incorporated
herein by reference.


                                       22
<PAGE>   23

PART IV

ITEM 14 - EXHIBITS, FINANCIAL STATEMENT SCHEDULES AND REPORTS ON FORM 8-K

(a)  The following documents are filed as a part of this report:

         (1) The Company's Financial Statements are incorporated herein by
         reference from the Company's 2000 Annual Report to Shareholders
         attached hereto as Exhibit 13. The specific items and the locations of
         such items are set forth below:

<TABLE>
<CAPTION>
Item                                                    Location
----                                                    --------

<S>                                                     <C>
Consolidated Statements of Income                       Annual Report under the caption "Biogen,
                                                        Inc. and Subsidiaries Consolidated
                                                        Statements of Income."

Consolidated Balance Sheets                             Annual Report under the caption "Biogen,
                                                        Inc. and Subsidiaries Consolidated Balance
                                                        Sheets."

Consolidated Statements of Cash Flows                   Annual Report under the caption "Biogen,
                                                        Inc. and Subsidiaries Consolidated
                                                        Statements of Cash Flows."

Consolidated Statements of Shareholders' Equity         Annual Report under the caption "Biogen,
                                                        Inc. and Subsidiaries Consolidated
                                                        Statements of Shareholders' Equity."

Notes to Consolidated Financial Statements              Annual Report under the caption "Biogen,
                                                        Inc. and Subsidiaries Notes to Consolidated
                                                        Financial Statements."

Report of Independent Accountants                       Annual Report under the caption "Report of
                                                        Independent Accountants."
</TABLE>

     With the exception of the portions of the Company's 2000 Annual Report to
Shareholders specifically incorporated herein by reference, such report shall
not be deemed filed as part of this Annual Report on Form 10-K.

     (2) The Company's Financial Statement Schedules, as required by Item 8 of
this Form 10-K, are incorporated herein by reference from the Company's 2000
Annual Report to Shareholders attached hereto as Exhibit 13. A list of such
Financial Statement Schedules is set forth below:

     Report of Independent Accountants on Financial Statement Schedule.
     Schedule II - Valuation and Qualifying Accounts and Reserves

     (3) Exhibits

                                       23
<PAGE>   24


Exhibit No.    Description
-----------    -----------

(3.1)          Articles of Organization, as amended (m)

(3.2)          By-Laws, as amended (f)

(4.1)          Form of Common Stock Share Certificate (h)

(4.2)          Certificate of Designation of Series A Junior Participating
               Preferred Stock (d)

(4.3)          Rights Agreement dated as of May 8, 1989 between the Registrant
               and The First National Bank of Boston, as Rights Agent (d)

(10.1)         Independent Consulting and Project Agreement dated as of June 29,
               1979 between the Registrant and Kenneth Murray (a)**

(10.2)         Letter Agreement dated September 11, 1998 with Kenneth Murray
               related to renewal of Independent Consulting Agreement (q) **

(10.3)         Minute of Agreement dated February 5, 1981 among the Registrant,
               The University Court of the University of Edinburgh and Kenneth
               Murray (a)**

(10.4)         Independent Consulting Agreement dated as of June 29, 1979
               between the Registrant and Phillip A. Sharp (a)**

(10.5)         Letter Agreement dated December 11, 1998 with Phillip A. Sharp
               related to chairmanship of Scientific Board and renewal of
               Independent Consulting Agreement (q)**

(10.6)         Project Agreement dated as of December 15, 1979 between the
               Registrant and Phillip A. Sharp (a)**

(10.7)         Share Restriction and Repurchase Agreement dated as of December
               15, 1979 between the Registrant and Phillip A. Sharp (a)**

(10.10)        Form of Amendment dated July 1, 1988 to Independent Consulting
               Agreement between the Registrant and Scientific Board Members
               (c)**

(10.11)        Letter regarding employment of James L. Vincent dated September
               23, 1985 (b)**

(10.12)        Letter agreement amending employment arrangement between the
               Registrant and James L. Vincent dated as of November 21, 1996
               (n)**

(10.13)        Form of Stock Option Agreement with James L. Vincent under 1985
               Non-Qualified Stock Option Plan (f)**


                                       24

<PAGE>   25


(10.14)        Form of Stock Option Agreement with James L. Vincent under 1985
               Non-Qualified Stock Option Plan (1995) (l)**

(10.15)        Form of Stock Option Agreement with James L. Vincent under 1985
               Non-Qualified Stock Option Plan (1997) (o)**

(10.17)        Form of Indemnification Agreement between the Registrant and each
               Director and Executive Officer (c)**

(10.18)        Cambridge Center Lease dated October 4, 1982 between Mortimer
               Zuckerman, Edward H. Linde and David Barrett, as Trustees of
               Fourteen Cambridge Center Trust, and B. Leasing, Inc. (a)

(10.19)        First Amendment to Lease dated January 19, 1989, amending
               Cambridge Center Lease dated October 4, 1982 (f)

(10.20)        Second Amendment to Lease dated March 8, 1990, amending Cambridge
               Center Lease dated October 4, 1982 (f)

(10.21)        Third Amendment to Lease dated September 25, 1991, amending
               Cambridge Center Lease dated October 4, 1982 (f)

(10.22)        Fourth Amendment to Lease dated October 6, 1993, amending
               Cambridge Center Lease dated October 4, 1982 (o)

(10.23)        Fifth Amendment to Lease dated October 9, 1997, amending
               Cambridge Center Lease dated October 4, 1982 (o)

(10.24)        Lease dated October 6, 1993 between North Parcel Limited
               Partnership and Biogen Realty Limited Partnership (i)

(10.25)        1983 Employee Stock Purchase Plan, as amended and restated
               through September 12, 1997 (o)**

(10.26)        1982 Incentive Stock Option Plan, as amended through June 15,
               2000 and restated, with form of Option Agreement (s)**

(10.27)        1985 Non-Qualified Stock Option Plan, as amended through June 15,
               2000 and restated (s)**

(10.28)        1987 Scientific Board Stock Option Plan, as amended through
               June 15, 2000 and restated (s)**

(10.29)        Voluntary Executive Supplemental Savings Plan (k)**

(10.30)        Amendment No. 1 dated April 25, 1997 to Voluntary Executive
               Supplemental Savings Plan (o)**

                                       25
<PAGE>   26

(10.31)        Amendment No. 2 dated March 11, 1998 to Voluntary Executive
               Supplemental Savings Plan (q)**

(10.32)        Amendment No. 3 dated September 27, 1999 to Voluntary Executive
               Supplemental Savings Plan (r) **

(10.33)        Amendment No. 4 dated December 13, 1999 to Voluntary Executive
               Supplemental Savings Plan (r)**

(10.34)        Amended and Restated Supplemental Executive Retirement Plan (o)**

(10.35)        Amendment No. 1 dated September 27, 1999 to Amended and Restated
               Supplemental Executive Retirement Plan (r), **

(10.36)        Voluntary Board of Directors Savings Plan  (k)**

(10.37)        Amendment No. 1 dated April 25, 1997 to Voluntary Board of
               Directors Savings Plan (o)**

(10.38)        Amendment No. 2 dated March 11, 1998 to Voluntary Board of
               Directors Savings Plan (q)**

(10.39)        Amendment No. 3 dated September 27, 1999 to Voluntary Board of
               Directors Savings Plan (r)**

(10.40)        Amendment No. 4 dated December 13, 1999 to Voluntary Board of
               Directors Savings Plan (r)**

(10.41)        Exclusive License and Development Agreement dated December 8,
               1979 between the Registrant and Schering Corporation (a)

(10.42)        Amendatory Agreement dated May 14, 1985 to Exclusive License and
               Development Agreement dated December 8, 1979 between the
               Registrant and Schering Corporation (b)

(10.43)        Amendment and Settlement Agreement dated September 29, 1988 to
               Exclusive License and Development Agreement dated December 8,
               1979 between the Registrant and Schering Corporation (f)

(10.44)        Amendment dated March 20, 1989 to Exclusive License and
               Development Agreement dated December 8, 1979 between the
               Registrant and Schering Corporation (f)

(10.45)        License Agreement (United States) dated March 28, 1988 between
               the Registrant and SmithKline Beecham Biologicals, s.a. (as
               successor to Smith Kline-R.I.T, s.a.) (f)

(10.46)        License Agreement (International) dated March 28, 1988 between
               the Registrant and SmithKline Beecham Biologicals, s.a. (as
               successor to Smith Kline-R.I.T., s.a.) (f)


                                       26

<PAGE>   27

(10.47)        Sublicense Agreement dated as of February 15, 1990 among the
               Registrant, SmithKline Beecham Biologicals, s.a (as successor to
               SmithKline Biologicals, s.a.) and Merck and Co., Inc. (f)

(10.48)        Supplemental Amendment and Agreement dated as of March 1, 1994
               between the Registrant and Schering Corporation (j)

(10.49)        Agreement and Amendment between the Registrant and Schering
               Corporation dated May 1, 1998 (p)

(10.50)        Letter agreement amending employment arrangement between the
               Registrant and James L. Vincent dated March 10, 2000 * **

(10.51)        Letter regarding employment of James C. Mullen dated March 18,
               1993 (r)**

(10.52)        Letter amending employment arrangement between the Registrant and
               James C. Mullen dated January 7, 1999 (r)**

(10.53)        Letter regarding employment of Burt Adelman, M.D. dated April 2,
               1996 (r)**

(10.54)        Letter regarding employment of Mark Leuchtenberger dated November
               14, 1996 (r)**

(10.55)        Letter agreement amending employment arrangement between the
               Registrant and Mark Leuchtenberger dated May 12, 1999 (r)**

(10.56)        Letter regarding employment of Thomas J. Bucknum, dated June 11,
               1999 * **

(10.57)        Letter agreement regarding employment arrangement between the
               Registrant and Cornelis Been, dated July 19, 1999 * **

(10.58)        Letter amending employment arrangement between the Registrant
               and James C. Mullen, dated May 3, 2000 * **

(13)           Incorporated portions of the Registrant's Financial Statements
               from its 2000 Annual Report to Shareholders *

(21)           Subsidiaries of the Registrant *

(23)           Consent of PricewaterhouseCoopers LLP *

-------------

     (a)  Previously filed with the Commission as an exhibit to the Registrant's
          Registration Statement on Form S-1, File No. 2-81689, and incorporated
          herein by reference.

     (b)  Previously filed with the Commission as an exhibit to the Registrant's
          Annual Report on Form 10-K for the fiscal year ended December 31,
          1985, as amended, File No. 0-12042, and incorporated herein by
          reference.

     (c)  Previously filed with the Commission as an exhibit to the Registrant's
          Annual Report on Form 10-K for the fiscal year ended December 31,
          1988, File No. 0-12042, and incorporated herein by reference.

                                       27

<PAGE>   28

     (d)  Previously filed with the Commission as an exhibit to the Registrant's
          Registration Statement on Form 8-A, File No. 0-12042, filed May 26,
          1989, and incorporated herein by reference.

     (e)  Previously filed with the Commission as an exhibit to the Registrant's
          Annual Report on Form 10-K for the fiscal year ended December 31,
          1991, File No. 0-12042, and incorporated herein by reference.

     (f)  Previously filed with the Commission as an exhibit to the Registrant's
          Annual Report on Form 10-K for the fiscal year ended December 31,
          1992, File No. 0-12042, and incorporated herein by reference.

     (g)  Previously filed with the Commission as an exhibit to the Registrant's
          Quarterly Report on Form 10-Q for the quarter ended June 30, 1993,
          File No. 0-12042, and incorporated herein by reference.

     (h)  Previously filed with the Commission as an exhibit to the Registrant's
          Registration Statement on Form S-3, File No. 33-51639 filed December
          21, 1993, and incorporated herein by reference.

     (i)  Previously filed with the Commission as an exhibit to the Registrant's
          Annual Report on Form 10-K for the fiscal year ended December 31,
          1993, File No. 0-12042, and incorporated herein by reference.

     (j)  Previously filed with the Commission as an exhibit to the Registrant's
          Quarterly Report on Form 10-Q for the quarter ended March 31, 1994,
          File No. 0-12042, and incorporated herein by reference.

     (k)  Previously filed with the Commission as an exhibit to the Registrant's
          Annual Report on Form 10-K for the fiscal year ended December 31,
          1994, File No. 0-12042, and incorporated herein by reference.

     (l)  Previously filed with the Commission as an exhibit to the Registrant's
          Annual Report on Form 10-K for the fiscal year ended December 31,
          1995, File No. 0-12042, and incorporated herein by reference.

     (m)  Previously filed with the Commission as an exhibit to the Registrant's
          Annual Report on Form 10-K for the fiscal year ended December 31,
          1996, File No. 0-12042, and incorporated herein by reference.

     (n)  Previously filed with the Commission as an exhibit to an amendment to
          the Registrant's Annual Report on Form 10-K/A for the fiscal year
          ended December 31, 1996, File No. 0-12042, and incorporated herein by
          reference.

     (o)  Previously filed with the Commission as an exhibit to the Registrant's
          Annual Report on Form 10-K for the fiscal year ended December 31,
          1997, File No. 0-12042, and incorporated herein by reference.


                                       28
<PAGE>   29


     (p)  Previously filed with the Commission as an exhibit to the Registrant's
          Quarterly Report on Form 10-Q for the quarter ended June 30, 1998,
          File No. 0-12042, and incorporated herein by reference.

     (q)  Previously filed with the Commission as an exhibit to the Registrant's
          Annual Report on Form 10-K for the fiscal year ended December 31,
          1998, File No. 0-12042, and incorporated herein by reference.

     (r)  Previously filed with the Commission as an exhibit to the Registrant's
          Annual Report on Form 10-k for the fiscal year ended December 31,
          1999, File No. 0-12042, and incorporated herein by reference

     *  Filed herewith

     ** Management contract or compensatory plan or arrangement

(b)  Reports on Form 8-K

     The Company did not file any reports on Form 8-K during the fourth quarter
of 2000.



                                       29
<PAGE>   30

                                   SIGNATURES

     Pursuant to the requirements of Section 13 or 15(d) of the Securities
Exchange Act of 1934, the Registrant has duly caused this report to be signed on
its behalf by the undersigned, thereunto duly authorized.

BIOGEN, INC.

By:       /s/ James C. Mullen
    ------------------------------------
      James C. Mullen, President and
      Chief Executive Officer

Dated March 30, 2001

     Pursuant to the requirements of the Securities Exchange Act of 1934, this
report has been signed below by the following persons on behalf of the
Registrant and in the capacities and on the dates indicated.

<TABLE>
<CAPTION>

SIGNATURES                     TITLE                                       DATE
----------                     -----                                       ----
<S>                            <C>                                         <C>
/s/ James C. Mullen            President, Chief Executive                  March 30, 2001
---------------------------    Officer and Director
James C. Mullen                (principal executive officer)


/s/ James L. Vincent           Chairman of the Board of                    March 30, 2001
---------------------------    Directors
James L. Vincent


/s/ Peter N. Kellogg           Vice President - Finance and Chief          March 30, 2001
---------------------------    Financial Officer (principal
Peter N. Kellogg               financial and accounting officer)


/s/ Alan Belzer                Director                                    March 30, 2001
---------------------------
Alan Belzer

/s/ Harold W. Buirkle          Director                                    March 30, 2001
---------------------------
Harold W. Buirkle

/s/ Mary L. Good               Director                                    March 30, 2001
---------------------------
Mary L. Good

/s/ Thomas F. Keller           Director                                    March 30, 2001
---------------------------
Thomas F. Keller

/s/ Roger H. Morley            Director                                    March 30, 2001
---------------------------
Roger H. Morley

/s/ Kenneth Murray             Director                                    March 30, 2001
---------------------------
Kenneth Murray
</TABLE>




<PAGE>   31

<TABLE>
<S>                            <C>                                         <C>
/s/ Phillip A. Sharp           Director                                    March 30, 2001
---------------------------
Phillip A. Sharp

/s/ Alan K. Simpson            Director                                    March 30, 2001
---------------------------
Alan K. Simpson

/s/ James W. Stevens           Director                                    March 30, 2001
---------------------------
James W. Stevens
</TABLE>
<PAGE>   32


                                  EXHIBIT INDEX


Exhibit No.       Description
-----------       -----------


(13)           Incorporated portions of the Registrant's Financial Statements
               from its 1999 Annual Report to Shareholders

(21)           Subsidiaries of the Registrant

(23)           Consent of PricewaterhouseCoopers LLP

(10.50)        Letter agreement amending employment arrangement between the
               Registrant and James L. Vincent dated March 10, 2000

(10.56)        Letter regarding employment of Thomas J. Bucknum, dated June 11,
               1999

(10.57)        Letter agreement regarding employment arrangement between the
               Registrant and Cornelis Been, dated July 19, 1999

(10.58)        Letter amending employment arrangement between the Registrant
               and James C. Mullen, dated May 3, 2000