Document:

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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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<S>                <C>
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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                                                                   Exhibit 10.22

CONSULTANT:                                            - Alan W. Dunton, M.D.
ACTIVBIOTICS CONTACT:                                  - Steven C. Gilman, Ph.D.
EFFECTIVE DATE:                                        - January 1, 2006

                             [ACTIVBIOTICS(R) LOGO]

                              CONSULTING AGREEMENT

     This Consulting Agreement (together with its attachments, this "Agreement")
made as of the date written above (the "Effective Date") is between
ActivBiotics, Inc., a Delaware corporation having an address at 110 Hartwell
Avenue, Lexington, MA 02421 (together with its affiliates and subsidiaries,
"ActivBiotics"), and the consultant named on the signature page ("Consultant")
ActivBiotics desires to have the benefit of Consultant's knowledge and
experience, and Consultant desires to provide Consulting Services (defined
below) to ActivBiotics, all as provided in this Agreement.

1 CONSULTING SERVICES. ActivBiotics retains Consultant and Consultant agrees to
provide Consulting Services to ActivBiotics (the "Consulting Services") as it
may from time to time reasonably request and as specified in the business terms
exhibit attached to this Agreement ("Business Terms Exhibit"). Any changes to
the Consulting Services (and any related compensation adjustments) must be
agreed upon in writing between Consultant and ActivBiotics prior to commencement
of the changes. Consultant understands and agrees that it is not the intent of
ActivBiotics that this Agreement or any actions of ActivBiotics be construed as
imposing any duty or obligation, express or implied, on Consultant to use,
purchase, prescribe, or recommend any product of ActivBiotics.

     1.1  PERFORMANCE. Consultant agrees to render the Consulting Services to
          ActivBiotics, or to its designee, (a) at such reasonably convenient
          times and places as ActivBiotics may direct, (b) under the general
          supervision of ActivBiotics, and (c) on a best efforts basis
          Consultant will comply with all rules, procedures and standards
          promulgated from time to time by ActivBiotics with regard to
          Consultant's access to and use of ActivBiotics' property, information,
          equipment and facilities. Consultant agrees to furnish ActivBiotics
          with written reports with respect to the Consulting Services if and
          when requested by ActivBiotics.

     1.2  THIRD PARTY CONFIDENTIAL INFORMATION. Consultant agrees not to use any
          trade secrets or other confidential information of any other person,
          firm, corporation, institution or other entity in connection with any
          of the Consulting Services.

     1.3  NO CONFLICTS. Consultant is under no contractual or other obligation
          or restriction which is inconsistent with Consultant's execution of
          this Agreement or the performance of the Consulting Services During
          the Term (defined below), Consultant will not enter into any
          agreement, either written or oral, in conflict with Consultant's
          obligations under this Agreement. Consultant will arrange to provide
          the Consulting Services in such manner

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          and at such times that the Consulting Services will not conflict with
          Consultant's responsibilities under any other agreement, arrangement
          or understanding or pursuant to any employment relationship Consultant
          has at any time with any third party.

     1.4  COMPLIANCE WITH POLICIES. If Consultant is a faculty member at or
          employee of a university or hospital ("Institution") 01 of another
          company, Consultant represents and warrants that pursuant to
          Institution's or company's policies concerning professional consulting
          and additional workload, Consultant is permitted to enter into this
          Agreement. If Consultant is required by Consultant's Institution to
          disclose to it any proposed agreements with industry, Consultant has
          made such disclosure. If Institution's prior approval of this
          Agreement is required by Institution policies, Consultant has obtained
          or will obtain and deliver to ActivBiotics, Institution's consent on
          the form attached to this Agreement prior to beginning the Consulting
          Services.

     1.5  ABSENCE OF DEBARRMENT. Consultant represents that neither Consultant
          nor any Consultant Personnel (defined below) has been debarred, and to
          the best of Consultant's knowledge, is not under consideration to be
          debarred, by the U.S. Food and Drug Administration from working in or
          providing consulting services to any pharmaceutical or biotechnology
          company under the Generic Drug Enforcement Act of 1992.

     1.6  CONSULTANT PERSONNEL. In the event that others are, or may hereafter
          become, associated with Consultant or are used by Consultant in
          connection with the Consulting Services ("Consultant Personnel"),
          Consultant agrees to procure from them agreements containing
          obligations substantially identical in form and substance to those in
          this Agreement, and Consultant agrees to cooperate with ActivBiotics
          in procuring execution by them of assignments and other papers as may
          be required by the terms of this Agreement.

2. COMPENSATION.

     2.1 PAYMENT. In consideration for the Consulting Services rendered by
Consultant to ActivBiotics, ActivBiotics agrees to pay Consultant the fees set
forth in the Business Terms Exhibit. Unless otherwise specified in the Business
Terms Exhibit, payments are due net thirty (30) days from ActivBiotics' receipt
of Consultant's invoice. Invoices will contain such detail as ActivBiotics may
reasonably require and will be payable in U S Dollars.

     2.2 EXPENSES. ActivBiotics will reimburse Consultant for all reasonable
travel and other expenses incurred by Consultant in rendering the Consulting
Services, provided that such expenses are consistent with ActivBiotics' Travel
and Expense Policy, are agreed upon in advance, and are confirmed by appropriate
written expense statements and other supporting documentation.

3. INVENTIONS.

     3.1  DEFINITION. Consultant will promptly disclose in confidence to
          ActivBiotics all inventions, discoveries, improvements, ideas,
          designs, processes, products, computer programs, works of authorship,
          databases, mask works, trade secrets, know-how, research and creations
          (whether or not patentable or subject to copyright or trade secret
          protection) that Consultant makes, conceives or reduces to
          practice, either alone or jointly with others, and that (a) result
          from the performance of the Consulting Services,

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          and/or (b) result from use of facilities, equipment, supplies,
          Research Materials (defined below), or Confidential Information
          (defined below) of ActivBiotics ("Inventions")

     3.2  OWNERSHIP. All Inventions will be the exclusive property of
          ActivBiotics. For purposes of the copyright laws of the United States,
          all Inventions will constitute "works made for hire", except to the
          extent such Inventions cannot by law be "works made for hire". To the
          extent Inventions have not been previously assigned to ActivBiotics,
          Consultant hereby assigns and, to the extent any such assignment
          cannot be made at present, hereby agrees to assign to ActivBiotics,
          without further compensation, all right, title and interest in and to
          all Inventions and any and all related patents, patent applications,
          copyrights, copyright applications, trademarks, trade names, trade
          secrets and other proprietary rights in the United States and
          throughout the world.

     3.3  RESEARCH MATERIALS. If the Consulting Services entail laboratory work
          or experiments, "Research Materials" means all materials furnished by
          ActivBiotics, all materials developed by Consultant in connection with
          the Consulting Services and all materials, the cost of which are
          reimbursed to Consultant by ActivBiotics hereunder. Research Materials
          include, in the case of biological materials, all progeny and
          unmodified derivatives of those materials, and in the case of chemical
          materials, all analogs, formulations, mixtures and compositions of
          those materials. Research Materials are the sole property of
          ActivBiotics and at ActivBiotics' option, are either to be disposed of
          or delivered to ActivBiotics upon request, but in any event within
          thirty (30) days after the expiration or termination of this Agreement
          Consultant agrees that Consultant will not use or evaluate Research
          Materials for any purpose other than as directed by ActivBiotics, nor
          transfer the Research Materials to any third party without the prior
          consent of ActivBiotics. Consultant will use the Research Materials in
          compliance with all laws and regulations. Consultant will be
          responsible for any claims or liabilities which arise as the result of
          Consultant's use, handling or storage of the Research Materials, and
          will hold ActivBiotics and its employees, officers and agents harmless
          from any loss, claim, damage or liability which may arise from or in
          connection with Consultant's use, handling or storage of Research
          Materials.

     3.4  RECORDS. Consultant will maintain adequate and current written records
          (in the form of notes, sketches, drawings and as may be specified by
          ActivBiotics), properly corroborated, to document the conception
          and/or first actual reduction to practice of any Invention. Those
          written records will be available to and remain the sole property of
          ActivBiotics at all times. If the Consulting Services entail
          laboratory work or experiments, the details of such work or
          experiments performed will be recorded in laboratory notebooks used
          only for recording work done on behalf of ActivBiotics. This will be
          of sufficient detail that other skilled persons, without reference to
          other material, could directly repeat the experiments. Results of all
          experiments will be documented, whether deemed to have been successful
          or not. All raw data will be included in the laboratory notebooks and
          will be available for inspection upon request. Data will be fully
          annotated such that cross-reference with records in laboratory
          notebooks is easily achieved. Laboratory notebooks (which will be the
          property of ActivBiotics) will be non-loose leaf and have numbered
          consecutive pages, the date to be filled in on each page. Consultant
          will sign and date each page at the end of each day on which work is
          performed and such signature will be countersigned or witnessed by
          another researcher each day.

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     3.5  AGREEMENT WITH INSTITUTION. This Agreement is made subject to the
          understanding that Consultant, if affiliated with an Institution, may
          be required to fulfill certain obligations, including teaching,
          directing laboratory operations, conducting research, and publishing
          work. It is further understood that Consultant may have signed an
          agreement concerning inventions with Institution, under which
          Consultant may be obligated to assign to Institution certain
          inventions which arise out of or otherwise relate to Consultant's work
          at or for Institution or from Consultant's use of certain of its
          facilities or intellectual property. In performing the Consulting
          Services, Consultant agrees not to utilize Institution facilities or
          intellectual property if the result of such use is that any Inventions
          will not be assignable solely to ActivBiotics. Use of Institution's
          telephone, fax machines or computers for communication purposes,
          however, will not constitute use of Institution's facilities under
          this Agreement.

     3.6  WORK AT THIRD PARTY FACILITIES. Consultant agrees not to make any use
          of any funds, space, personnel, facilities, equipment or other
          resources of a third party in performing the Consulting Services not
          take any other action that would result in a third party owning or
          having a right in any Inventions, unless agreed upon in writing in
          advance by ActivBiotics.

4. CONFIDENTIAL INFORMATION.

     4.1  DEFINITION. "Confidential Information" means all trade secrets and
          confidential or proprietary information owned, possessed or used by
          ActivBiotics, learned of by Consultant or developed by Consultant in
          connection with the Consulting Services, whether or not labeled
          "Confidential", including but not limited to (a) all Inventions,
          marketing plans, business strategies, scientific data, financial
          information, forecasts, personnel information and customer lists of
          ActivBiotics, (b) all information of third parties that ActivBiotics
          has an obligation to keep confidential, and (c) the terms and
          conditions of this Agreement (including the compensation paid to
          Consultant pursuant to Section 2). Confidential Information does not
          include information which (i) is in the public domain or which becomes
          part of the public domain through no wrongful act on Consultant's part
          but only after it becomes so publicly known, (ii) is already in
          Consultant's possession at the time of disclosure by ActivBiotics,
          other than by previous disclosure by ActivBiotics, as evidenced by
          written or electronic records, or (iii) that becomes known to
          Consultant through disclosure by a third party having the right to
          disclose the information, as evidenced by written or electronic
          records.

     4.2  OBLIGATIONS OF CONFIDENTIALITY. During the Term and thereafter,
          Consultant will not directly or indirectly publish, disseminate or
          otherwise disclose, use for Consultant's own benefit or for the
          benefit of a third party, deliver or make available to any third party
          any Confidential Information, other than in furtherance of the
          purposes of this Agreement, and only then with the prior written
          consent of ActivBiotics. If required, Consultant may disclose the
          Confidential Information to a governmental authority or by order of a
          court of competent jurisdiction, provided that such disclosure is
          subject to all applicable governmental or judicial protection
          available for like material and reasonable advance notice is given to
          ActivBiotics. Consultant will exercise all reasonable precautions to
          physically protect the integrity and confidentiality of the
          Confidential Information and will not remove any Confidential
          Information or copies thereof from

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          ActivBiotics' premises except to the extent necessary to fulfill the
          Consulting Services, and then only with ActivBiotics' prior consent.
          Consultant may disseminate or permit access to Confidential
          Information only to Consultant Personnel who have a need to know such
          Confidential Information in the course of the performance of their
          duties and who are bound to protect the confidentiality of the
          Confidential Information consistent with the terms of this Agreement.

5. PUBLICATION. Consultant agrees to submit to ActivBiotics a copy of any
proposed manuscript or other materials to be published or otherwise publicly
disclosed which contains information or any discussion relating to ActivBiotics
or the Consulting Services, in sufficient time to enable ActivBiotics to
determine if patentable Inventions or any Confidential Information of
ActivBiotics would be disclosed Consultant will cooperate with ActivBiotics in
this respect and will delete from the manuscript or other disclosure any
Confidential Information if requested by ActivBiotics, and will assist
ActivBiotics in filing for patent protection for any patentable Inventions prior
to publication or other disclosure.

6. TERM AND TERMINATION.

     6.1  TERM. This Agreement will commence on the Effective Date and continue
          for the term specified on the Business Terms Exhibit (the "Term"),
          unless sooner terminated pursuant to the express terms of this Section
          6 or extended by mutual agreement of the parties.

     6.2  TERMINATION FOR BREACH. If either party breaches in any material
          respect any of its material obligations under this Agreement, in
          addition to any other right or remedy, the non-breaching party may
          terminate this Agreement in the event that the breach is not cured
          within thirty (30) days after receipt by that party of written notice
          of the breach.

     6.3  TERMINATION BY ACTIVBIOTICS. ActivBiotics may terminate this Agreement
          (a) immediately at any time upon written notice to Consultant in the
          event of a breach of this Agreement by Consultant which cannot be
          cured (i.e. breach of the confidentiality obligation) and/or (b) at
          any time without cause upon not less than thirty (30) days' prior
          written notice to Consultant.

     6.4  EFFECT OF EXPIRATION/TERMINATION. Upon expiration or termination,
          neither ActivBiotics nor Consultant will have any further obligations
          under this Agreement, except (a) the liabilities accrued through the
          date of termination, and (b) the obligations under, sections 3, 4, 6
          and 7 will survive. Upon expiration or termination, and in any case
          upon ActivBiotics' request, Consultant will return immediately to
          ActivBiotics all tangible Confidential Information, including all
          copies and reproductions thereof, except for one (1) copy which may be
          retained solely for archival purposes.

7. MISCELLANEOUS.

     7.1  INDEPENDENT CONTRACTOR. All Consulting Services will be rendered by
          Consultant as an independent contractor and this Agreement does not
          create an employer-employee relationship between ActivBiotics and
          Consultant. Consultant will have no rights to receive any employee
          benefits, such as health and accident insurance, sick leave or
          vacation which are accorded to regular ActivBiotics employees.
          Consultant will not in any way represent himself to be an employee,
          partner, joint venturer, or agent of ActivBiotics.

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     7.2  TAXES. Consultant will pay all required taxes on Consultant's income
          from ActivBiotics under this Agreement. Consultant will provide
          ActivBiotics with Consultant's taxpayer identification number or
          social security number, as applicable

     7.3  ASSIGNABILITY AND BINDING EFFECT. The Consulting Services to be
          rendered by Consultant are personal in nature. Consultant may not
          assign or transfer this Agreement or any of Consultant's rights or
          obligations hereunder except to a corporation of which Consultant is
          the sole stockholder. In no event will Consultant assign or delegate
          responsibility for actual performance of the Consulting Services to
          any other natural person except to Consultant Personnel as provided
          for under this Agreement. This Agreement will be binding upon and
          inure to the benefit of the parties and their respective legal
          representatives, heirs, successors and permitted assigns.

     7.4  HEADINGS. The section headings are included solely for convenience of
          reference and will not control or affect the meaning or interpretation
          of any of the provisions of this Agreement.

     7.5  NOTICES. Any notices or other communications from one party to the
          other will be in writing and will be given by addressing the same to
          the other at the address or facsimile number set forth in this
          Agreement. Notices to ActivBiotics will be marked "ATTENTION: CHIEF
          FINANCIAL OFFICER". Notice will be deemed to have been duly given when
          (a) deposited in the United States mail with proper postage for first
          class Registered or Certified Mail prepaid, return receipt requested,
          (b) sent by any reputable commercial courier, delivery confirmation
          requested, (c) delivered personally, or (d) if promptly confirmed by
          mail or commercial courier as provided above, when dispatched by
          facsimile.

     7.6  NO MODIFICATION. This Agreement may be changed only by a writing
          signed by authorized representatives of both parties.

     7.7  SEVERABILITY. In the event that any one or more of the provisions
          contained in this Agreement will, for any reason, be held to be
          invalid, illegal or unenforceable in any respect, such invalidity,
          illegality or unenforceability will not affect any other provisions of
          this Agreement, and all other provisions will remain in full force and
          effect. If any provision of this Agreement is held to be excessively
          broad, it will be reformed and construed by limiting and reducing it
          so as to be enforceable to the maximum extent permitted by law.

     7.8  ENTIRE AGREEMENT. This Agreement constitutes the entire agreement of
          the parties with regard to its subject matter, and supersedes all
          previous written or oral representations, agreements and
          understandings between the parties.

     7.9  GOVERNING LAW. This Agreement will be governed by, and construed and
          enforced in accordance with, the laws of the Commonwealth of
          Massachusetts applicable to contracts made and to be performed
          therein, without giving effect to the principles thereof relating to
          the conflict of laws.

     7.10 COUNTERPARTS. This Agreement may be executed in any number of
          counterparts, each of which will be deemed an original, but all of
          which together will constitute one and the same instrument.

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     IN WITNESS WHEREOF, the parties hereto have duly executed this Agreement
under seal as of the Effective Date.

                               ACTIVBIOTICS INC.

                               By: /s/ Steven C. Gilman
                                   ---------------------------------------------
                               Name:  Steven C. Gilman, Ph.D.
                               Title: President and Chief Executive Officer

                               CONSULTANT:

                               By: /s/ Alan W. Dunton
                                   ---------------------------------------------
                               Name: Alan W. Dunton, MD

                               Address: 80 Tanglewylde Avenue
                                        Bronxville, NY 10708

                               Telephone: (917)859-5191
                               FAX: (914) 961-7663

                               Tax ID/SS No. (redacted)
                                             -----------------------------------
                                             (required for payment)

                                       7

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                             BUSINESS TERMS EXHIBIT

1.   CONSULTING SERVICES:

     Consulation on business or legal proceedings outside the normal course of
     efforts as a Board Member and Chairman of the Board of ActivBiotics, Inc

2.   COMPENSATION:

     As full compensation for the Consulting Services, ActivBiotics will pay
     Consultant $250.00 per hour ($2000/day), not to exceed $25,000 during the
     Term, without ActivBiotics' specific written consent. [A full day will
     consist of a minimum of eight (8) hours of Consulting Services.]

     On the last day of each calendar month, Consultant will invoice
     ActivBiotics for Consulting Services rendered and travel expenses incurred
     during the preceding month.

     Invoices should reference this Agreement and be forward to:

     ActivBiotics, Inc
     Attn: Accounts Payable Dept.
     110 Hartwell Avenue
     Lexington, MA 02421

3.   TERM:

     This Agreement will be for an initial term of one (1) year beginning on the
     Effective Date, and may be extended for additional periods, at
     ActivBiotics' option and with Consultant's consent.

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