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                                                               Exhibit 10.25

[ACTIVBIOTICS LOGO]

June 13, 2006

James L. Warren
48 Groton Road
N. Chelmsford, MA 01863

Dear Jim,

     Following our recent discussions about your termination from the Company,
this letter contains the complete terms and conditions under which the Company
would provide certain benefits as described below.

     1. RESIGNATION. Your last day of work will be Wednesday, June 14, 2006 (the
"Last Work Day"). As of the Last Work Day, you resign any positions of the
Company that you may hold or have held and the Company accepts such resignations
and relieves you of any responsibilities, obligations or duties associated with
such positions after the Last Work Day.

     2. CONSULTING AGREEMENT. To assist in the transition of your
responsibilities to other employees, the Company agrees to enter into a
consulting relationship with you. Under the terms of a separately negotiated
Consulting Agreement, the company will compensate you for services provided
under the contract You agree to continue to adhere to the terms and conditions
of your Invention and Non-Disclosure Agreement with the Company, Company work
policies, this letter Agreement, and any Consulting Agreement.

     3. STOCK OPTIONS. With regard to all of your Stock Option Agreements with
the Company, and only for purposes of those Stock Option Agreements, the Company
agrees that the date of termination of your association with the Company shall
be deemed to be the last date (the "Last Consulting Day") as defined under a
separate Consulting agreement You agree to waive any right your might otherwise
have to continued vesting of stock options after the Last Consulting Day, and
you acknowledge that your vested stock options will remain exercisable for
twelve (12) months following Last Consulting Day You acknowledge the sufficiency
of the this extension of exercise time as consideration for your continued
adherence to the terms and conditions of your Invention and Non-Disclosure
Agreement and for your agreements in this letter Agreement to release certain
claims against the Company.

     4. WAIVERS AND RELEASES. (a) You agree that the compensation and benefits
paid to you through the Termination Date, together with vacation pay for unused
vacation accrued as of the Termination Date (which the Company will pay to you
after the Termination Date in accordance with applicable law), constitute full
and complete satisfaction of any and all sums which are now or might hereafter
have become owing to you for services rendered by you during your employment
with the Company or in connection with the termination of your employment with
the Company.

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(b) This letter agreement is not intended to and does not constitute an
admission of liability by either you or the Company, and shall not be used as
evidence of liability or wrongdoing on the part of either you or the Company.

(c) You forever give up, waive and release any and all claims, charges,
complaints, grievances or promises of any and every kind you may have up to the
date of this letter agreement or that may arise after the date of this letter
agreement against the Company, its shareholders, directors, officers and
employees, and related or affiliated persons, including, without limitation--

          (i)  your rights under Title VII of the Civil Rights Act of 1964, as
               amended by the Civil Rights Act of 1991, M.G.L. c 151B, the
               Employee Retirement Income Security Act ("ERISA"), the Equal Pay
               Act, the Americans with Disabilities Act ("ADA"), the Age
               Discrimination in Employment Act ("ADEA") and other federal and
               state statutes prohibiting discrimination on the basis of age,
               gender, sexual orientation, race, religion, national origin or
               handicap;

          (ii) any claims for breach of employment contract (whether express or
               implied); and

          (iii) any common law claims, including without limitation, claims for
               wrongful termination, defamation, invasion of privacy,
               intentional or negligent infliction of emotional distress,
               interference with contractual or advantageous business relations,
               personal injury, breach of contract, unpaid wages or
               compensation, or claims for unreimbursed expenses

     However, for clarity, this waiver and release shall not apply to any claims
that you may have for breach of this letter agreement

     5. CONFIDENTIALITY. You agree that this letter agreement is confidential
and that you will not disclose the fact that it exists or its terms and
conditions with anyone else except your immediate family members, attorney, tax
accountant, or as required by law, and that the individuals to whom disclosure
is made under this paragraph agree to similarly maintain the confidentiality of
this letter agreement

     6. NON-DISPARAGEMENT. You agree that you will not make any disparaging
comments, statements or other communications about the Company, its
shareholders, directors, officers, employees, related or affiliated persons, or
business practices, and that you will not otherwise do or say anything that
could disrupt the good morale of the employees of the Company or otherwise harm
its business or reputation. The Company agrees that its employees who managed
you during your employment with the Company, its officers and directors, and its
other employees and agents to the extent ActivBiotics can control the actions of
such employees and agents, will not make any disparaging comments, statements or
other communications about you or your employment with the Company. The Company
agrees that, in accordance with Company policy, it will refer all inquiries
regarding your employment to its Human Resources department, and the Human
Resources department will only confirm dates of employment and positions held.
For clarity, this Paragraph 6 shall not apply to any truthful statement that any
person is required to make in any legal proceeding, or pursuant to any
governmental or regulatory investigation

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     7. RETURN OF PROPERTY. You understand and agree that you must return all
Company property in your possession or control to the Company on or before the
Termination Date, including but not limited to, all building access cards, keys,
parking permits, laptop computers, dial-in access cards, documents, materials
and information relating to the business of the Company, and any other Company
property.

     8. ADHERENCE TO INVENTION AND NON-DISCLOSURE AGREEMENT: You agree to abide
by any and all employment agreements entered into between you and the Company
prior to or during your employment with the Company, including without
limitation the Invention and Non-Disclosure Agreement.

     9. CONSEQUENCES OF BREACH. In the event that you breach this letter
agreement, any other agreement with the Company (including without limitation
the Invention and Non-Disclosure Agreement with the Company) or Company work
policies, or if you otherwise behave in any way unprofessionally in relationship
to the Company, in addition to whatever legal or equitable remedy may be
available in compensation of that injury, the Company may halt payments of
compensation and provision of benefits under this letter agreement and demand
the return from you of the consideration paid pursuant to this letter agreement,
and you agree to immediately repay all such amounts. If the Company seeks an
equitable remedy, you agree that you will not oppose the granting of such relief
on the grounds that an award of damages would provide an adequate remedy.

     10. GENERAL PROVISIONS. If any term of this letter agreement, or the
application of any such term to any person or circumstance, is or would be, to
any extent, invalid or unenforceable, the remainder of this letter agreement, or
the application of such term to persons or circumstances other than those to
which it is or would be invalid or unenforceable, shall not be affected, and
each term of this letter agreement shall be valid and be enforced to the fullest
extent permitted by law.

     This letter agreement has been made in Massachusetts and Massachusetts law
(other than choice of law provisions) applies to it. You consent to the
exclusive jurisdiction of Massachusetts courts for any dispute or proceeding
concerning this letter agreement.

     Together with the Invention and Non-Disclosure Agreement and any stock
option agreements between you and the Company, this letter agreement sets forth
the entire agreement between you and the Company concerning its subject matter.
You acknowledge that no one has promised you anything that is different from
what is set forth in this letter agreement. No other promises or agreements
shall be binding upon the Company with respect to the subject matter of this
letter agreement unless separately agreed to in writing.

     11. CONSULTATION WITH AN ATTORNEY; REVOCATION. As required by the Older
Workers Benefit Protection Act of 1990, if you are over the age of 40, you
acknowledge and agree that--

          (i)  you have been advised and given the opportunity to consult with
               your own counsel prior to signing this letter agreement;

          (ii) you have been given up to forty-five (45) days from the receipt
               of this letter agreement to consider whether to sign it and that,
               if you execute this letter agreement prior to the end of this
               period, you acknowledge and agree that the prior execution was a
               knowing and voluntary waiver of

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           your right to consider this letter agreement for a full forty-five
           (45) days, and was due to your conclusion that you had ample time
           within which to consider and understand this letter agreement, and
           within which to review this letter agreement with your counsel;

     (iii) you has been advised that even after you sign this letter agreement,
           you may revoke it within seven (7) days of the date of this signing,
           by delivering a signed revocation notice by certified mail to: Cathy
           Carew, ActivBiotics, Inc, 110 Hartwell Ave, Lexington, MA 02421; and

     (iv)  this letter agreement shall not become effective and in force until
           eight (8) days after you sign it (provided then that you have not
           revoked it).

ACTIVBIOTICS, INC.


By: /s/ Steven C. Gilman                06-13-2006
    ---------------------------------   Date
    Steven C. Gilman, Ph D.
    Chief Executive Officer


AGREED:


/s/ James L. Warren                      06-13-2006
-------------------------------------   Date
James L. Warren