February 22, 2008

Seth Tasker
39 Dartmouth Street
Arlington, MA 02474

Dear Seth,

On behalf of Infinity Pharmaceuticals, Inc. (the “Company”), I am pleased to
offer you the position of Corporate Counsel, initially reporting to Gerald
Quirk.

1.
Effective Date: The effective date of your full-time employment with the Company
shall be Monday, March 17, 2008.

2.
Salary: Your base salary will be $5,076.93 per biweekly pay period (equivalent
to $132,000 on an annualized basis). In addition, in accordance with the
Company’s regular compensation practices, you will receive, approximately
annually, a salary review, and the Company may adjust your salary based on your
performance, the Company’s performance, and/or such other factors as may be
determined at the sole discretion of the Company’s Board of Directors or its
designee.

3.
Sign-on Bonus: The Company will pay you a bonus of $7,000.00 on the date of the
first paycheck following commencement of your full-time employment. Should you
terminate for any reason within 12 months of your starting date after having
received your bonus, the Company reserves the right to seek repayment of all or
a pro-rata portion of your bonus.

4.
Contingent Compensation: In addition to your salary and benefits, you may be
eligible to participate in the Infinity Contingent Compensation program. This
program may result in a cash bonus, depending on your and the Company’s
achievements of goals and objectives, as well as overall business conditions.
The Contingent Compensation program is administered by the Company's Board of
Directors in their sole discretion. In order to be eligible for any type of
payment under the program, you must be actively employed by the Company at the
time the payment is made.

5.
Benefits: You may participate in any and all of the benefit programs that the
Company establishes and makes available to its employees from time to time,
provided you are eligible under (and subject to all provisions of) the plan
documents governing these programs.

6.
Vacation: Upon your date of hire, you will start to accrue vacation time at a
rate of 15 days per year, which may be taken in accordance with Company policy;
12 paid holidays annually will be observed.

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Page 2
February 22, 2008
Seth Tasker

7.
Equity Participation, Vesting of Stock: Subject to approval by the Company’s
Board of Directors, you will be granted a stock option exercisable for 6,000
shares of the Company’s Common Stock. A complete description of the terms and
conditions of the stock option award will be contained in the Infinity
Pharmaceuticals 2000 Stock Incentive Plan and the form of stock option agreement
to be entered into by you and the Company. The option will vest as to one fourth
(1/4) of the shares on the first anniversary of your commencement of full-time
employment with the Company and as to one forty-eighth (1/48) of the shares
monthly thereafter until all shares are vested, provided that you remain
employed by the Company. In addition, in accordance with the Company’s
compensation practices, you will receive, approximately annually, a merit stock
review which shall be based on your performance, the Company’s performance, and
other such factors as may be determined by the Company’s Board of Directors.

8.
Employment At-Will: Your employment with the Company will be at-will, meaning
that you will not be obligated to remain employed by the Company for any
specified period of time and the Company will not be obligated to continue your
employment for any specific period. Both you and the Company may terminate the
employment relationship, with or without cause, at any time, with or without
notice. Similarly, nothing in this letter shall be construed as an agreement,
either express or implied, to pay you any compensation or grant you any benefit
beyond the end of your employment with the Company (except as described herein).

9.
Proprietary Information, No Conflicts: As a condition of employment, you agree
to execute the Company’s standard form of Invention, Non-Disclosure, and
Non-Competition Agreement and to be bound by all of the provisions thereof. You
hereby represent that you are not presently bound by any employment agreement,
confidential or proprietary information agreement, or similar agreement with any
current or previous employer that would impose any restriction on your
acceptance of this offer or that would interfere with your ability to fulfill
the responsibilities of your position with the Company.

10.
Employment Eligibility Verification: Please note that all persons employed in
the United States are required to complete an Employment Eligibility
Verification Form on the first day of employment and to submit an original
document or documents that establish identity and employment eligibility within
three business days of employment.

11.
Successors and Assigns: This letter of offer will be binding upon and inure to
the benefit of the Company’s successors and assignees. In the event of a merger
or consolidation (whether or not the Company is the surviving or the resulting
corporation), the surviving or resulting corporation will be bound by the
obligations set forth in this letter offer.

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Page 3
February 22, 2008
Seth Tasker

Seth, all of us here at Infinity are very enthusiastic about your commitment to
joining the Company and have the highest expectation of your future
contributions.

Please indicate your understanding and acceptance of the foregoing terms of your
employment by signing the enclosed copy of this letter and returning it to
Melanie Getchell no later than February 27, 2008. After that date, the offer
will expire.

Very truly yours,

/s/Adelene Q. Perkins

Adelene Q. Perkins
Executive Vice President and Chief Business Officer

The foregoing correctly sets forth the terms of my at-will employment by
Infinity Pharmaceuticals, Inc.

 
/s/Seth A. Tasker     2/27/2008
Seth Tasker                                 Date