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                            [BIOGEN, INC. LETTERHEAD]

                                                                   EXHIBIT 10.55

                                                      May 12, 1999

Mr. Mark Leuchtenberger
20 Old Farm Road
Newton, MA 02459

Dear Mark:

On behalf of Biogen, I am pleased to outline the details of a proposed
secondment to the Paris office, whereby you will assume the position of Vice
President - International. This secondment is intended to be a minimum of two
years in duration, after which you will be repatriated back to Boston,
Massachusetts. At the end of the two-year period, an option to extend for
additional time will be by mutual agreement only.

As this is an international secondment, your employment is subject to foreign
government entry documents and work visas and your acceptance of the terms and
conditions outlined in this letter. Your foreign secondment will begin as soon
as the appropriate documentation can be completed and approved. Your point of
origin will be Boston, Massachusetts.

Your compensation and benefits package is designed to provide you with a level
of income and benefits that is comparable with individuals at the same level and
experience as yourself working in the United States. We have also taken into
consideration the additional costs you may reasonably anticipate as a result of
living in France.

The terms and conditions outlined in this letter will be in effect for the
period of your secondment. At the conclusion of this secondment and upon your
return to the United States, the provisions of the letters will cease and you
will return to United States compensation and benefits programs.

COMPENSATION:

Your base salary for this position will be increased to $ 245,000 (paid in US
dollars) effective May 17, 1999. Typically, we can arrange to have your salary
paid in local currency and/or in US dollars, for your convenience.

You will continue to participate in the Executive Bonus Plan with a maximum
bonus payment of 30% of base pay. The plan payments are determined by a
combination of corporate and individual results.

GOODS AND SERVICES ALLOWANCE

In addition to your base salary, you will receive a goods and services allowance
in addition to base salary if the cost of living at the location is in excess of
comparable United States costs. Such cost relationships are measured by indices
established by an independent consulting firm engaged by Biogen. Human Resources
will establish the allowance with data provided by the consulting firm. This
allowance is tied to income level and family size. Your 1999 allowance will be
$3,250 per month.


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Mr. Mark Leuchtenberger
May 12, 1999

This allowance is paid on a monthly basis along with base salary and will be
reviewed and possibly adjusted annually (or more frequently, in the event of a
significant change) due to changes in host country costs and /or currency
values.

HOUSING

Based upon our conversations, it was decided that it is in the best interest of
Biogen and yourself for you to retain ownership of your US residence during your
period of expatriation. You will be expected to maintain your current residence
here in the US while on foreign assignment. You will be using your Newton
residence and your personal vehicle during trips to Cambridge. The company will
provide an apartment for you while you are in Paris. The company agrees to
provide a management firm to watch over your Newton home while you are on
foreign assignment. This will include mail forwarding, bill paying, and walk
throughs. If repairs are to be made to your home, you will be financial
responsible for such repairs.

TAX EQUALIZATION

Our philosophy is such that the Company will provide you with an after-tax
income equivalent to that which you would have received had you remained in the
US. This is done by hypothetical US Federal and actual state tax deductions from
your total salary. Your withholding tax may also fluctuate according to changes
in compensation or tax rates.

A tax advisor from a well-respected accounting firm selected by the Company will
be made available to help you prepare your U.S. and foreign income tax returns;
the fees for these services will be borne by the Company. Filings of tax returns
and payment of tax owed in the US or abroad is entirely your responsibility and
the Company assumes no responsibility for your taxes.

Additionally, appropriate and reasonable tax consulting sessions with the
accounting firm selected by the Company will be provided before and after your
secondment.

DOCUMENT ASSISTANCE:

The Company will advise you on appropriate documentation and will pay for those
and other associated costs incurred in obtaining papers for travel to, or
establishing residence in the France and/or countries to which you will be
travelling in conjunction with the secondment abroad.

CULTURAL ORIENTATION

We will engage our relocation firm to provide you both a preview of the new
country that you will visit as well as a cultural orientation.

TRANSFER ALLOWANCE:

Upon leaving the United States, you will be paid a one-time payment of $5,000 to
pay for such things as appliance purchases, telephone installation, auto
registration, driver's license, and other personal and miscellaneous expenses
not specifically mentioned in this letter.


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Mr. Mark Leuchtenberger
May 12, 1999

SHIPMENT OF PERSONAL HOUSEHOLD GOODS:

We will move personal goods and furniture up to 5,000 pounds to France.
Additional items can be stored at the company's expense. All items to be moved
would be fully insured by the Company. You will also be entitled to store
personal and household goods.

Biogen will also pay for any import duties and other expenses necessary for
actual delivery of these goods. This does not include certain unusual or special
items such as automobiles, antiques, trailers, campers, boats, etc. The
selection of the storage company and the carrier will be at the discretion of
the Company.

AUTO SALES AND LEASE:

Biogen will not pay for shipment of automobiles to the foreign location. If you
decide to sell your present vehicle(s), we will reimburse you for the costs
associated with the sale.

While on foreign secondment, the Company will provide a leased automobile for
you. Due to your unique situation, you will be provided a Mercedes E320. The
costs of the leased vehicle and insurance will be at the company expense; the
cost of fuel and repairs will be your expense. Should you choose to want to
lease a second automobile, the company will arrange for the vehicle to be leased
through the company, at your expense.

VACATION:

You will be eligible for vacation under the U.S. policy.

HOLIDAYS:

You will be eligible to observe the local policy for holidays in France.

EMPLOYEE BENEFITS:

You shall continue to be eligible to receive benefits that, in general, are
substantially similar to those that US employees receive. For your medical
coverage, you will be in the Tufts out of area plan. Your children in the US
will continue to be covered locally through COBRA insurance.

To the extent you are required by local laws and regulations to participate in
Foreign benefit plans which are in addition to those you would have normally
received in the US, the Company will pay for your contributions. Where possible
the Company will help you obtain a waiver from participation in such local
benefit plans programs.

LANGUAGE INSTRUCTION:

If required, the Company will pay for reasonable language instruction to be
arranged upon your arrival in France.

EMERGENCY LEAVE:

The Company will grant up to seven days off plus travel time for personal
emergencies such as serious illness or death of an immediate family member.
Round trip air transportation will be granted.


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Mr. Mark Leuchtenberger
May 12, 1999


HOME LEAVE:

Biogen encourages you and your family to return to the United States to renew
business, professional, personal, and family contacts on an annual basis. For
that purpose, the Company will provide the following home leave back to the
United States during each twelve-month period of the secondment:

-    A period of time above and beyond eligible vacation time, not to exceed
     fourteen calendar days or ten working days will be granted.
-    Actual living expenses will be your responsibility.
-    Expenses for the rental of a car in the United States will be reimbursed
     for a maximum period of fourteen days.
-    Pay in any form will not be granted in lieu of home leave, nor will payment
     be authorized if travel is not to the point of origin.

Additionally, based upon your unique situation, you and Jim have agreed that
your children will be entitled to 5 visits to Paris during the year. In order to
manage the expenses related to these trips, these trips will be reimbursed at a
30-day advanced nonrefundable coach rate. Additionally, it is expected that with
the extensive travel you and the children have, you will use frequent flyer
miles accumulated toward these trips, before requesting reimbursement from
Biogen. Additionally, you, Tracey and Sara Grace will be reimbursed for 1
roundtrip for each 12-month period. Also, there is an expectation that certain
business functions (such as the year end holiday party) will require the
attendance of your and Tracey. The company agrees to reimburse you for such
trips.

TERMINATION:

In the event of involuntary termination, the Company will arrange to return you
to the United States. This will also include your household and personal
effects, limited to the same weight and size of your shipment at the beginning
of your secondment. The expenses will be borne by the Company only if the move
out of France occurs within thirty days of termination.

You agree that all rights and obligations in the event of an involuntary
termination shall be determined based on the laws of the US and the Commonwealth
of Massachusetts, and you hereby waive any rights you may have under the laws
and regulations of France and the European Union.

REPATRIATION:

Although an expected secondment duration of approximately two years has been
planned, further extensions may be negotiated. However, after two years, should
you remain in the France, your overall compensation situation will be reviewed.
If it is determined that your status will not remain that of an expatriate, you
will be considered a local national and specifically, the following
allowance/benefits may be discontinued:

-    Commodities and Service Allowance
-    Company arranged housing
-    Company paid home leave
-    Tax equalization


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Mr. Mark Leuchtenberger
May 12, 1999

Approximately six months before conclusion of your secondment, discussions will
begin regarding your repatriation and a suitable position back in the United
States upon successful completion of the foreign secondment. Every attempt will
be made to find a position within the Company which suitably utilizes your
skills and experience and which fits your career interests. At that point, I
will generate a letter that will detail all terms and conditions of your
repatriation.

On behalf of Jim Mullen, it is with great pleasure that I present the terms and
conditions of your appointment. I trust that you will find your experiences in
your new capacity in Paris to be professionally rewarding, and that all the
daily experiences of living in France will be personally fulfilling for you.
Please countersign and date your acceptance below.

                                        Sincerely,

                                        /s/ Richard W. Fisher

                                        Richard W. Fisher
                                        Director - Human Resources


Accepted: /s/ Mark Leuchtenberger            Date: 9/10/99
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