Prepared by MERRILL CORPORATION





Exhibit 10.13


Effective as of May 15, 2001


Ms. Tara Clark

26 Hearthstone Place

Andover, MA  01810


Dear Tara:


I am writing to confirm our offer to you
for a position as Vice President, Marketing, reporting to Walter Ogier,
President and COO.  We are pleased to
offer you a starting salary of $10,903.75 per month (equivalent to $130,845.00
per year), to be paid semi-monthly and reviewed annually.  The monthly salary includes an allowance to
help offset contributions to certain benefits should you elect them.  You are also eligible for an annual cash
bonus of 20% of base salary and to participate in our annual merit-based stock
option program, provided that any award of options will be subject to the
approval of our Board of Directors. You are also entitled to receive shares
of BTI common stock in accordance with the terms and conditions of the Eligix,
Inc. Management Equity Incentive Plan. In addition, you will receive medical
and dental insurance, 15% of the cost of which is contributed by the employee.  Term life insurance, equivalent to one times
your annual salary, and parking will be provided by the company without
employee contribution. Long-term disability insurance is provided, 100% of the
cost is contributed by the employee (your annual premium would be approximately
$767.00).  You will also be entitled to
three weeks paid vacation accrued on an annual basis and 11 paid holidays per
year.  In the event your employment is
terminated by Biotransplant without cause, you would receive up to 6 months
base salary, payable in 12 equal semi-monthly installments which would be
discontinued upon your securing other employment.


Your employment is subject to your
signing an Invention, Non-Disclosure and Non-Compete Agreement with
BioTransplant, in the form previously provided to you, and review of any
agreement you may have with others to insure that your employment with
BioTransplant is not in conflict with any such agreements.


Also, this offer is contingent upon your
ability to provide proof of your legal right to work in the United States, as
defined by Federal regulations.


By signing this letter below, you
acknowledge that the foregoing offered terms of your employment with
BioTransplant do not constitute (i) a significant diminution of your duties,
responsibilities, power, title or office in effect at Eligix, Inc. immediately
prior to your acceptance of this offer or (ii) a material reduction in your
compensation (including without limitation, salary, bonuses, options and
benefits) or material change in your manner of compensation (including without
limitation, the timing of any salary or bonus payments) in effect for you at
Eligix, Inc. immediately prior to your acceptance of this offer.





If you agree to the terms outlined above,
please sign both copies of this letter and return one copy using the enclosed,
stamped envelope.


We look forward to your joining
BioTransplant. We are confident that you will make a significant contribution
to BioTransplant's future success.


Sincerely,


/s/ Elliot Lebowitz

Elliot Lebowitz, Ph.D.

President and CEO


Accepted and Agreed to:


	/s/
  Tara Clark	 	 
	

	 	 
	Name:  Tara Clark	 	Date:  May 15, 2001