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                                                                   Exhibit 10.15

[ACTIVBIOTICS LOGO]

                                                                 October 7, 2004

Glenn M. Kazo
280 Page Hill Road
New Ipswich NH 03071

Dear Glenn,

I am pleased to offer you the position of Vice President and Chief Business
Officer with ActivBiotics, Inc., with a start date of November 5, 2004. Your
2005 gross base salary will be $250,000 payable in accordance with the company's
standard payroll practices, which currently provides that such salary will be
paid in bi-weekly installments of $9,615.88. In addition, you will receive a
signing bonus in the amount of $20,000, payable in the first pay period in
calender year 2005. Beginning in calendar year 2005, you will also be eligible
to participate in our bonus program and you may earn up to an additional 30% of
your annual base salary. The bonus is at the discretion of the Board of
Directors of ActivBiotics and will be determined by meeting individual
performance objectives, as well as, company and management team performance
objectives.

Another important component of our compensation program is participation in the
ActivBiotics' 2001 Equity Incentive Plan, as amended (the "Plan"). ActivBiotics
will grant an option exercisable for 580,000 shares of common stock of
ActivBiotics (the "Option"). This Option, including the exercise price, the
number of shares, and the vesting schedule, thereof, is subject to the approval
of ActivBiotics' Board of Directors. The Option will be subject to the terms and
conditions of the Plan and the Stock Option Agreement executed pursuant to the
Plan (the "Option Agreement"). The Option may include, among other things, a
4-year vesting schedule where 25% of the Option's shares vest on the first year
anniversary of the date of grant of the Option, and 25% of the Option's shares
vest annually thereafter, provided you remain employed with ActivBiotics on each
applicable vesting date. As an additional incentive, ActivBiotics proposes to
accelerate the vesting schedule by allowing 50% of the shares vesting in your
first year of employment to vest immediately at the close of the first research
partnership or out-licensing transaction you successfully complete on behalf of
the company. The remaining 50% of the shares vesting in your first year of
employment will vest on the first anniversary of the date of grant of the
Option. Due to the long-term contributions your deals may have on the value of
ActivBiotics, in the event of your termination of employment with ActivBiotics,
the company may extend the time to exercise the Option, subject to approval by
the Board of Directors, in its sole discretion. For further information, please
consult the form of Stock Option Agreement and the Plan, each of which is
enclosed herein. Any additional options which may be granted in the future will
be subject to the applicable option agreement and the option plan then in
effect. Glenn, this Option award represents an important equity position in
ActivBiotics, and we look forward to working together to continue to build the
value of this company.

At the discretion of the board, you will be eligible to participate in any and
all bonus and benefit programs that the Company establishes and makes available
to its employees, including

     -    Health and dental insurance

     -    Long and short term disability insurance and long term care insurance

     -    8 fixed and 4 floating holidays

     -    401K retirement plan

     -    Direct deposit of payroll (optional)

     -    Section 125 plan, allowing pre-tax deduction of health and dental
          insurance contributions

     -    4 weeks (20 days) paid vacation time per year.

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Glenn, to help you with the commuting issues during your first six months of
employment, ActivBiotics agrees to reimburse you for two nights of local lodging
during weeks you are not traveling on business. In your seventh month of
employment and beyond, ActivBiotics agrees to reimburse you for one night per
week of local lodging during weeks you are not traveling on business. We
understand that you will work out of your home office on most days that you will
be required to travel. If at any time during the first 24 months of your
employment with ActivBiotics, you decide to move your home residence in order to
be closer to work, you will be eligible for ActivBiotics' usual and customary
relocation assistance, which includes up to 6% realtor fees on the new property,
as well as reasonable costs to pack and move your household goods.

Your employment with ActivBiotics will be on an "at-will" basis, which means
that either you or the company may terminate the employment relationship at any
time, for any or no reason, with or without Cause, and with or without prior
notice. If you resign from ActivBiotics or your employment is terminated by
ActivBiotics with Cause within 24 months of your start date, you will be
required to repay ActivBiotics a pro rata portion of your signing bonus, as well
as, a pro rata portion of the relocation assistance, if any was provided by
ActivBiotics. If your employment is terminated by ActivBiotics without Cause,
you will be eligible to receive 6 months salary continuation payments at your
base salary rate in effect as of your date of termination, paid in accordance
with the company's standard payroll practices then in effect ActivBiotics'
obligation to pay any salary continuation is conditioned upon the company
receiving a general release from you in a form and substance reasonable
satisfactory to ActivBiotics.

For purposes of this letter, "Cause" shall mean any one or more of the
following: (i) your substantial and continuing failure, for within a period of
30 days after receiving notice thereof from the Board of Directors or the Chief
Executive Officer of ActivBiotics, to perform your responsibilities to the
company; (ii) your willful misconduct, fraud, or material dishonesty that
results in material harm to the company; (iii) your breach of fiduciary duty to
the company; (iv) your deliberate disregard of the rules or policies of the
company or directions from the Board of Directors or the Chief Executive Officer
of ActivBiotics, which results in material harm to the company; (v) your
commission of an act which constitutes unfair competition with the company or
which induces any customer or supplier to breach a contract with the company; or
(vi) your material breach of the Invention and Non-Disclosure agreement or any
other agreement executed by you with the company.

Except as set forth in this or other applicable agreements, ActivBiotics shall
have no other obligations to you upon the cessation of your employment other
than payment of any accrued but unused vacation through the termination date.
All payments and benefits described herein will be subject to applicable
federal, state and local tax withholdings.

The company requires you to verify that the performance of the position at
ActivBiotics does not and will not breach any agreement entered into by you
prior to employment with ActivBiotics (i.e., you have not entered into any
agreements with previous employers that are in conflict with your obligations to
ActivBiotics). Please provide us with a copy of any potentially conflicting
agreements for our review. You will also be required to sign the company's
standard Invention and Non-Disclosure agreement, which is enclosed, as a
condition of your employment with the company.

Although you will devote all of your business time, attention and energies to
the business and interests of the Company, the Company agrees that you shall be
permitted to (i) serve as a member of the board of directors of another company
upon approval of a majority of the Board, (ii) serve as an officer or director
of such charitable organizations or trade associations as you shall from time to
time deem appropriate, and (iii) continue to maintain your position on the Board
of Directors for Prolong Pharmaceuticals, provided, (a) the time spent on such
activities does not infringe in any material respect upon the time required by
you to perform your duties or is otherwise incompatible with those duties (b)
service as an officer or director does not conflict with any policy of the
Company or has a material adverse impact on the Company's ability to obtain or
maintain directors and officers insurance coverage.

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Glenn, we are confident that you will make an important contribution to the
success of ActivBiotics, and look forward to working with you. This offer will
be valid until October 8, 2004 please do not hesitate to contact me if you have
any questions. In order to accept the position as offered, please sign this
letter and the attached application for employment for our records, and return
both to me.

Sincerely,

                                         I accept the offer of employment as
ACTIVBIOTICS, INC.                       outlined above,


/s/ Cathy Carew                          /s/ Glenn M. Kazo
--------------------------------------   ---------------------------------------
By: Cathy Carew,                         Glenn M. Kazo
Title: Vice President, Human Resources