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                                                                   Exhibit 10.21

[ACTIVBIOTICS LOGO]

April 28, 2006

David McLachlan
51 Brentwood Road
Chelmsford, MA 01824

Dear Mr. McLachlan:

On behalf of the Board of Directors of ActivBiotics (the "Company"), I am
pleased to offer you a position as a member of the Board of Directors, effective
March 8, 2006

As an independent member of the Board, we offer you the following compensation
package:

CASH COMPENSATION: $10,000 annual compensation as a Board Member, plus an
additional $7,500 as a stipend for Audit Committee Chair, payable in 4 quarterly
installments.

STOCK OPTIONS: You will receive an initial grant of stock options to purchase
35,000 shares of the Company's Common Stock (equivalent to .17% of outstanting
shares on an as converted, fully diluted basis) at an exercise price equal to
the Fair market value of the Company's Common Stock on the date of the grant.
Your option grants will vest in equal monthly installments over a four-year
period from the date of grant, so long as you continue to serve as a Director.

EXPENSES: The Company will reimburse you for all reasonable and necessary
out-of-pocket expenses incurred by you in connection with the performance of
your duties as a Director.

INDEMNIFICATION: The Company shall, to the fullest extent permitted Delaware
General Corporation law as then in effect, indemnify you if as a result of your
position as a director of the Company you are involved in any manner (including,
without limitation, as a party or a witness) or are threatened to be made so
involved in any threatened, pending or completed investigation, claim, action,
suit or proceeding, whether civil, criminal, administrative or investigative
(including, without limitation, any action, suit or proceeding by or in the
right of the Company to procure a judgment in its favor) (a "Proceeding")
against all expenses (including reasonable attorneys' fees), judgments, fines
and amounts paid in settlement actually and reasonably incurred by you in
connection with such Proceeding. Such indemnification shall be a contract right
and shall include the right to receive payment in advance of any expenses
incurred by you in connection with such Proceeding.

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ActivBiotics, Inc. 110 Hartwell Avenue Lexington, MA02421 781-372-4800 Fax 781-274-8638
                              www.activbiotics.com
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INSURANCE: The Company is covered by Directors and Officers Liability and
Reimbursement Insurance issued by Carolina Casualty Company with a $5,000,000
liability limit.

David, ActivBiotics has an exciting future but of course faces many challenges
in realizing its ultimate potential. We can only achieve our vision for the
Company with the help individuals like you who have a significant breath and
depth of experience, demonstrated talent and personal commitment. To that end, I
welcome you to join the ActivBiotics Board of Directors.

Sincerely,


/s/ Alan W. Dunton
-------------------------------------
Alan W. Dunton, MD
Chairman, ActivBiotics

I hereby accept my election as a member of the Board of Directors of
ActivBiotics, effective January 27, 2006


/s/ David McLachlan
-------------------------------------
David McLachlan


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