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                                                                   EXHIBIT 10.42


[BIOGEN LOGO]                                    VICE PRESIDENT, HUMAN RESOURCES
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FOURTEEN CAMBRIDGE CENTER, CAMBRIDGE, MA 02142 * 617-679-2000 * FAX 617-679-3595

                                 June 21, 2000


Mr. Peter N. Kellogg
3617 Dartmouth Avenue
Dallas, Texas 75205


Dear Peter:

This letter supercedes my letter dated June 19 and represents Jim Mullen's offer
to you on behalf of Biogen. You are offered the position of Vice President and
Chief Financial Officer of Biogen at an annual salary of $325,000. You will
report directly to Jim Mullen and you will become a member of the Biogen
Operating Team. Consistent with Biogen's compensation policy, you will be
eligible for a pro-rated merit salary review at year-end 2000 and annually
thereafter. You will also be eligible to participate in Biogen's annual
incentive compensation plan with a 50% target bonus and assuming an August 1,
2000 start date, Biogen will guarantee a pro-rated target bonus of $68,250 for
year 2000. Your principal place of employment will be at Biogen's headquarters
in the Cambridge, Massachusetts area.

Upon employment you will receive $200,000 as a one-time special cash bonus,
which will be treated as an interest-free forgivable loan. The bonus will be
forgiven over a thirty-six month, straight-line schedule. In the event you
voluntarily terminate your employment, or Biogen terminates your employment for
poor performance or cause, prior to the thirty-six months from the date of the
loan, the unforgiven portion of the loan would be payable to Biogen within six
months of your termination date. If your employment is involuntarily terminated
for non-cause within the first three years, or there is a change of control that
results in your termination, the full balance of this loan will be forgiven.

As of the date you commence employment, you will be granted an option to
purchase 200,000 shares of the common stock of Biogen, Inc. The exercise price
per share will be the average of the high and low sale prices as reported in the
National Association of Securities Dealers Automated Quotation System ("NASDAQ")
on the date you commence employment. The option will have a ten-year term and
vest over a five-year period at the rate of 20% per year starting on the
anniversary of the date you commence employment. Based on your performance you
will be eligible for a merit stock option grant at year-end 2000 pursuant to
Biogen's merit stock option policy. This grant will be comparable with awards
granted to the three most senior executive officers reporting to Jim Mullen.

You will be able to choose from a menu of benefit options through our flexible
benefits program, BioChoice. These include health care, life, dependent life,
and disability insurance as well as two flexible spending accounts, Med-Flex and
D-Flex, for eligible medical or dependent-care expenses. At the beginning of the
calendar quarter following your date of hire you will be eligible to participate
in Biogen's pension and 401(k) savings plans. Biogen also offers a variety of
additional benefits including childcare and elder care referral service,
educational matching gifts, credit union, and group



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[BIOGEN LOGO]                                    VICE PRESIDENT, HUMAN RESOURCES
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Mr. Peter N. Kellogg
June 21, 2000
Page 2


homeowners and automobile insurance. You will receive more detailed information
regarding your benefits on your first day of employment.

Biogen will provide relocation benefits to facilitate your move from Dallas,
Texas. A relocation counselor will contact you to discuss the relocation
program, and will send you a package detailing the relocation policy to you.
These benefits will include the reimbursement of many of the costs associated
with both the sale of your current house and the purchase of a new house in the
Boston area; temporary living; return trips home; home finding; and movement of
your household goods. Please read the policy carefully as it will outline the
parameters of these benefits and detail the reimbursement process.

In addition, Biogen will also provide the following supplemental relocation
benefits:

-    BRIDGE LOAN: We will provide a bridge loan on the equity of your home in
     Dallas, which will be in place for up to six months, if necessary. We will
     consider an extension beyond this period if unusual market conditions do
     not allow the sale of your home.

-    MORTGAGE LOAN: Biogen will also provide you a fully subordinated
     interest-free mortgage loan of $1,000,000 (secured by real property of your
     choice and with prompt substitution of security permitted on your
     reasonable request, providing that the security shall consist of your
     principal residence in the Boston area once you acquire one). The mortgage
     loan will be repayable to Biogen the sooner of five years from your date of
     employment or eighteen months following the point in time the pre-tax
     profit of your vested stock options exceeds $5,000,000. In the event that
     the pre-tax profit of your vested options does not exceed $5,000,000 five
     years from your date of employment, the payment date of the mortgage loan
     will be extended for up to two years following your fifth anniversary date.

-    MORTGAGE REIMBURSEMENT: If necessary, Biogen will reimburse you for the
     mortgage and other reasonable costs related to your property in Dallas for
     up to six months. This reimbursement will commence after you have purchased
     a home in the Boston area and will end on the earlier of (i) six months, or
     (ii) when you have closed on the sale of your Dallas property.

If you voluntarily terminate your employment or if your employment is terminated
by Biogen for cause, you will be required to repay Biogen the entire amount of
the loan the sooner of six months following the termination of your employment
or the closing on the sale of the property by which the loan is then secured. If
your employment is involuntarily terminated for non-cause within the first five
years, or there is a change of control that results in your termination, your
loan will be forgiven. The Company has the right to renegotiate this change of
control clause if there is a new change of control provision, but will guarantee
that it will be no less favorable than the agreement set forth in this
employment letter.



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[BIOGEN LOGO]                                    VICE PRESIDENT, HUMAN RESOURCES
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Mr. Peter N. Kellogg
June 21, 2000
Page 3


As an officer of the Company, you will be required to participate in our program
for executive stock ownership. We believe it is very important that our key
leaders have personal stock ownership. I will outline the program to you.

Additionally, in your new role you will be provided the following:

-    VACATION: You are entitled, during your employment with Biogen, to four (4)
     weeks vacation, accrued on a monthly pro-rated basis.

-    SUPPLEMENTAL SAVINGS PLAN: You are entitled, during your employment with
     Biogen, to participate in the Voluntary Executive Supplemental Savings
     Plan. This plan allows you to defer from your current taxation up to 50% of
     your base salary and 100% of your bonus, if there is one. I can outline the
     details of the program if you are interested in participating.

-    LIFE INSURANCE: You will be provided, during your employment with Biogen,
     Biogen's Executive Term Life Insurance coverage for a total of $1,000,000.
     This coverage is based on your successfully meeting the medical standards
     as stated in the Executive Term insurance policy.

-    TAX REVIEW/PREPARATION: You are entitled, during your employment with
     Biogen, to the preparation and/or review, including review of estimated
     taxes of your annual Federal and State tax returns, which is currently
     administered through Price WaterhouseCoopers. The cost of this service is
     covered by Biogen. In the event you choose to continue the services
     provided by your current tax advisor, Biogen will reimburse you the cost of
     these services up to but not exceeding the amount incurred by Biogen
     through Price WaterhouseCoopers.

-    INVOLUNTARY TERMINATION: If your employment with Biogen is terminated by
     Biogen without cause, Biogen will protect you by paying you a supplementary
     amount (the "Supplementary Amount") equal to your then present base salary
     for a period (the "Extra Period") ending on the earlier of (i) the date
     twelve months from your termination and (ii) the date you start another
     job. During such period, Biogen will also pay to continue your health
     benefits (i.e., health and dental plan coverage), provided such benefits
     are accorded employees generally and Biogen can obtain the relevant
     coverage. If you need continued coverage to prevent a gap in health
     coverage between your Biogen coverage and that at your new job, Biogen will
     extend such coverage for up to 30 days (to the extent that the Extra Period
     is less than twelve months) after you start your new job. The Supplementary
     Amount will be paid on the same schedule as your salary would have been
     paid. You will not be an employee of Biogen during the time of such
     payments and will not accrue any benefits or other rights (such as, but not
     limited to, pension plan vesting or accrual, stock option



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[BIOGEN LOGO]                                    VICE PRESIDENT, HUMAN RESOURCES
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Mr. Peter N. Kellogg
June 21, 2000
Page 4


     vesting, vacation pay, etc.) during such period except health benefits as
     described above. You agree to notify us when you accept a new job.

Commencement of employment with Biogen is contingent on the satisfactory
completion of a pre-employment drug screening test at least five business days
prior to your start date. We require all new employees to sign an Intellectual
Property/Confidentiality Agreement on the first day of employment. You will also
be required to complete a medical history review with our occupational health
department, which is scheduled after your first day of employment.

It is Biogen's policy to comply with federal, state and local guidelines
applicable to its facilities and to take all reasonable steps to ensure the
health and safety of Biogen employees. Consistent with this policy, your normal
duties may require you from time to time to attend meetings or perform functions
in any or all of Biogen's facilities.

The Federal Government requires you to provide proper identification verifying
your eligibility to work in the United States. Please bring the appropriate
identification with you on your first day of employment.

On behalf of Jim Mullen, we very much look forward to your positive response and
to your joining us as early as practical. We are confident that you will make a
significant contribution to Biogen's future success.

Please confirm your acceptance by signing this offer letter and noting your
preferred start date. I also invite you to complete the enclosed invitation to
self-identify. Please complete the employment application and the drug screen
authorization forms and then return both signed documents to me in the enclosed
self addressed, stamped envelope. The other original offer letter is for your
records.


                                        Sincerely,



                                        /s/ Frank A. Burke, Jr.
                                        ------------------------------------
                                        Frank A. Burke, Jr.


FAB:bk
Attachments

cc: Mr. James C. Mullen



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[BIOGEN LOGO]                                    VICE PRESIDENT, HUMAN RESOURCES
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Mr. Peter N. Kellogg
June 21, 2000
Page 5


I am pleased to accept the offer of employment described above:

ACCEPTED:


/s/ Peter N. Kellogg                                               8/7/00
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Peter N. Kellogg                                            Preferred Start Date