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                                                                    Exhibit 10.6

                               ACTIVBIOTICS, INC.

                        2006 EMPLOYEE STOCK PURCHASE PLAN

     The following constitute the provisions of the 2006 Employee Stock Purchase
Plan of ActivBiotics, Inc.

1.   PURPOSE

     The purpose of the Plan is to provide employees of the Company and its
Designated Subsidiaries with an opportunity to purchase Common Stock of the
Company. It is the intention of the Company to have the Plan qualify as an
"Employee Stock Purchase Plan" under Section 423 of the Code. The provisions of
the Plan shall, accordingly, be construed so as to extend and limit
participation in a manner consistent with the requirements of that section of
the Code.

2.   DEFINITIONS

     2.1. BOARD means the Board of Directors of the Company.

     2.2. CODE means the Internal Revenue Code of 1986, as amended.

     2.3. COMMON STOCK means the common stock, par value $0.001 per share, of
the Company.

     2.4. COMPANY means ActivBiotics, Inc., a Delaware corporation.

     2.5. COMPENSATION means all regular straight time compensation including
commissions but shall not include payments for overtime, shift premium,
incentive compensation, incentive payments, bonuses and other irregular or
infrequent compensation or benefits.

     2.6. CONTINUOUS STATUS AS AN EMPLOYEE means the absence of any interruption
or termination of service as an Employee. Continuous Status as an Employee shall
not be considered interrupted in the case of (i) sick leave; (ii) military
leave; (iii) any other leave of absence approved by the Administrator, provided
that such leave is for a period of not more than 90 days, unless reemployment
upon the expiration of such leave is guaranteed by contract or statute, or
unless provided otherwise pursuant to Company policy adopted from time to time;
or (iv) in the case of transfers between locations of the Company or between the
Company and its Designated Subsidiaries.

     2.7. CONTRIBUTIONS means all amounts credited to the account of a
participant pursuant to the Plan.

     2.8. CORPORATE TRANSACTION means a merger or consolidation of the Company
with and into another person or the sale, transfer, or other disposition of all
or substantially all of the Company's assets to one or more persons (other than
any wholly-owned subsidiary of the Company) in a single transaction or series of
related transactions.

     2.9. DESIGNATED SUBSIDIARIES means the Subsidiaries which have been
designated by the Board from time to time in its sole discretion as eligible to
participate in the Plan.

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     2.10. EMPLOYEE means any person, including an Officer, who is customarily
employed for at least twenty (20) hours per week and more than five (5) months
in a calendar year by the Company or one of its Designated Subsidiaries.

     2.11. EXCHANGE ACT means the Securities Exchange Act of 1934, as amended.

     2.12. OFFERING COMMENCEMENT DATE means the first business day of each
Offering Period of the Plan.

     2.13. OFFERING PERIOD means any of the periods, generally of six (6) months
duration, as set forth in Section 4.

     2.14. OFFICER means a person who is an officer of the Company within the
meaning of Section 16 of the Exchange Act and the rules and regulations
promulgated thereunder.

     2.15. OFFERING TERMINATION DATE means the last business day of each
Offering Period of the Plan.

     2.16. PLAN means this Employee Stock Purchase Plan.

     2.17. PURCHASE PRICE means with respect to an Offering Period an amount
equal to eighty five percent (85%) of the Fair Market Value (as defined in
Section 7.2 below) of a Share on the Offering Commencement Date or on the
Offering Termination Date, whichever is lower; provided, however, that (i) if
there is an increase in the number of Shares available for issuance under the
Plan as a result of a stockholder-approved amendment to the Plan, (ii) all or a
portion of such additional Shares are to be issued with respect to the Offering
Period underway at the time of such increase ("Additional Shares"), and (iii)
the Fair Market Value of a Share of Common Stock on the date of such increase
(the "Approval Date Fair Market Value") is higher than the Fair Market Value on
the Offering Commencement Date for such Offering Period, then in such instance
the Purchase Price with respect to Additional Shares shall be eighty five
percent (85%) of the Approval Date Fair Market Value or the Fair Market Value of
a Share of Common Stock on the Offering Termination Date, whichever is lower.

     2.18. SHARE means a share of Common Stock, as adjusted in accordance with
Section 18 of the Plan.

     2.19. SUBSIDIARY means a corporation, in an unbroken chain of corporations
beginning with the Company if, at the time of the granting of the option, each
of the corporations other than the last corporation in the unbroken chain owns
stock possessing fifty percent (50%) or more of the total combined voting power
of all classes of stock in one of the other corporations in such chain.

3.   ELIGIBILITY

     3.1. Any person who is an Employee as of the Offering Commencement Date of
a given Offering Period shall be eligible to participate in such Offering Period
under the Plan, subject to the requirements of Sections 5.1 and the limitations
imposed by Section 423(b) of the Code.

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     3.2. Any provisions of the Plan to the contrary notwithstanding, no
Employee shall be granted an option under the Plan (i) if, immediately after the
grant, such Employee (taking into account stock which would be attributed to
such Employee pursuant to Section 424(d) of the Code) would own capital stock of
the Company and/or hold outstanding options to purchase stock possessing five
percent (5%) or more of the total combined voting power or value of all classes
of stock of the Company or of any Subsidiary of the Company, or (ii) if such
option would permit his or her rights to purchase stock under all employee stock
purchase plans (described in Section 423 of the Code) of the Company and its
Subsidiaries to accrue at a rate which exceeds Twenty-Five Thousand Dollars
($25,000) of the Fair Market Value (as defined in Section 7.2 below) of such
stock (determined at the time such option is granted) for each calendar year in
which such option is outstanding at any time.

4.   OFFERING PERIODS

     Each Offering Period will begin on January 1 or July 1 and end on the next
following June 30 or December 31, respectively, with the first Offering Period
commencing on January 1, 2007. At any time and from time to time, the Board may
change the duration and/or the frequency of Offering Periods with respect to
future Offering Periods or suspend operation of the Plan with respect to
Offering Periods not yet commenced.

5.   PARTICIPATION

     5.1. An eligible Employee may become a participant in the Plan by
completing a subscription agreement on the form provided by the Company and
filing it with the Company's payroll office prior to the applicable Offering
Commencement Date, unless a later time for filing the subscription agreement is
set by the Board for all eligible Employees with respect to a given Offering
Period. The subscription agreement shall set forth the percentage of the
participant's Compensation (subject to Section 6.1 below) to be paid as
Contributions pursuant to the Plan.

     5.2. Payroll deductions shall commence on the first payroll following the
Offering Commencement Date and shall end on the last payroll paid on or prior to
the Offering Termination Date of the Offering Period to which the subscription
agreement is applicable, unless sooner terminated by the participant as provided
in Section 10.

6.   METHOD OF PAYMENT of CONTRIBUTIONS

     6.1. A participant may elect to have payroll deductions made on each payday
during any Offering Period in an amount not less than one percent (1%) and not
more than fifteen percent (15%) (or such other percentage as the Board may
establish from time to time before an Offering Commencement Date) of such
participant's Compensation on each payday during the Offering Period. All
payroll deductions made by a participant shall be credited to his or her account
under the Plan. A participant may not make any additional payments into such
account.

     6.2. A participant may discontinue his or her participation in the Plan as
provided in Section 10. In addition, if the Board has so announced to Employees
at least five (5) days prior to the scheduled beginning of the next Offering
Period to be affected by the Board's determination, a participant may, on one
occasion only during each Offering Period, change the rate of his or her
Contributions with respect to the Offering Period by completing and filing with
the Company a new subscription agreement authorizing a change in the payroll
deduction rate. If otherwise permitted, no such change shall enable a
participant to resume Contributions other than as of an

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Offering Commencement Date, following a withdrawal of Contributions during an
Offering Period pursuant to Section 10. Any such change in rate shall be
effective as of the first payroll period following the date of filing of the new
subscription agreement, if the agreement is filed at least ten (10) business
days prior to such period and, if not, as of the second following payroll
period.

     6.3. Notwithstanding the foregoing, to the extent necessary to comply with
Section 423(b)(8) of the Code and Section 3.2 herein, a participant's payroll
deductions may be decreased during any Offering Period scheduled to end during
the current calendar year to zero percent (0%). Payroll deductions reduced to
zero percent (0%) in compliance with this Section 6.3 shall re-commence
automatically at the rate provided in such participant's subscription agreement
at the beginning of the first Offering Period which is scheduled to end in the
following calendar year, unless terminated by the participant as provided in
Section 10.

7.   GRANT OF OPTION

     7.1. On the Offering Commencement Date of each Offering Period, each
eligible Employee participating in such Offering Period shall be granted an
option to purchase on the Offering Termination Date of that Offering Period a
number of Shares determined by dividing such Employee's Contributions
accumulated prior to such Offering Termination Date and retained in the
participant's account as of the Offering Termination Date by the applicable
Purchase Price. However, the maximum number of Shares an Employee may purchase
during each Offering Period shall be________Shares(1), and provided further that
such purchase shall be subject to the limitations set forth in Sections 3.2 and
12.

     7.2. The fair market value of the Company's Common Stock on a given date
(the "Fair Market Value") means the value of a share of Common Stock on a
particular date determined by such methods or procedures as may be established
by the Committee. Unless otherwise determined by the Committee, the Fair Market
Value of the Common Stock as of any date (or, in the event that the Common Stock
is not traded on such date, on the immediately preceding trading date), is the
closing price for the Common Stock as reported by the National Association of
Securities Dealers Automated Quotation ("Nasdaq") System (or on any other
national securities exchange on which the Common Stock is then listed) for that
date or, if no closing price is reported for that date, the closing price on the
next preceding date for which a closing price was reported.

8.   EXERCISE OF OPTION

     Unless a participant withdraws from the Plan as provided in Section 10, his
or her option for the purchase of Shares will be exercised automatically on the
Offering Termination Date of an Offering Period, and the maximum number of full
Shares subject to the option will be purchased at the applicable Purchase Price
with the accumulated Contributions in his or her account. No fractional Shares
shall be issued. The Shares purchased upon exercise of an option hereunder shall
be deemed to be transferred to the participant on the Offering Termination Date.
During his or her lifetime, a participant's option to purchase Shares hereunder
is exercisable only by him or her.

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(1)  This number will represent [twenty-five] percent (|25|%) of the total
     allocable pool under the Plan.

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9.   DELIVERY

     As promptly as practicable after each Offering Termination Date of each
Offering Period, the Company shall arrange the delivery to each participant, as
appropriate, of a certificate representing the Shares purchased upon exercise of
his or her option. Any payroll deductions accumulated in a participant's account
which are not sufficient to purchase a full Share shall be retained in the
participant's account for the subsequent Offering Period, subject to earlier
withdrawal by the participant as provided in Section 10 below. Any other amounts
left over in a participant's account after an Offering Termination Date shall be
returned to the participant.

10.  VOLUNTARY WITHDRAWAL; TERMINATION OF EMPLOYMENT

     10.1. A participant may withdraw all but not less than all of the
Contributions credited to his or her account under the Plan at any time prior to
each Offering Termination Date by giving written notice to the Company. All of
the participant's Contributions credited to his or her account will be paid to
him or her promptly after receipt of his or her notice of withdrawal and his or
her option for the current Offering Period will be automatically terminated, and
no further Contributions for the purchase of Shares will be made during the
Offering Period.

     10.2. Upon termination of the participant's Continuous Status as an
Employee prior to the Offering Termination Date of an Offering Period for any
reason, including retirement or death, the Contributions credited to his or her
account will be returned to him or her or, in the case of his or her death, to
the person or persons entitled thereto under Section 14, and his or her option
will be automatically terminated.

     10.3. In the event an Employee fails to remain in Continuous Status as an
Employee of the Company for at least twenty (20) hours per week during the
Offering Period in which the employee is a participant, he or she will be deemed
to have elected to withdraw from the Plan and the Contributions credited to his
or her account will be returned to him or her and his or her option terminated.

     10.4. A participant's withdrawal during an Offering Period will not have
any effect upon his or her eligibility to participate in a succeeding Offering
Period or in any similar plan which may hereafter be adopted by the Company.

11.  INTEREST

     No interest shall accrue on the Contributions of a participant in the Plan.

12.  STOCK

     12.1. Subject to adjustment as provided in Section 18, the maximum number
of Shares which shall be made available for sale under the Plan shall be________
(2)Shares. If the Board determines that, on a given Offering Termination Date,
the number of shares with respect

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(2). This number will represent [one and a half] percent (|1.5|%) of the Company
     on a fully diluted basis immediately after the closing of the IPO.

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to which options are to be exercised may exceed (i) the number of shares of
Common Stock that were available for sale under the Plan on the Offering
Commencement Date, or (ii) the number of shares available for sale under the
Plan on such Offering Termination Date, the Board may in its sole discretion
provide that the Company shall make a pro rata allocation of the Shares
available for purchase on such Offering Commencement Date or Offering
Termination Date, as applicable, in as uniform a manner as shall be practicable
and as it shall determine in its sole discretion to be equitable among all
participants exercising options to purchase Common Stock on such Offering
Termination Date. The Company may make pro rata allocation of the Shares
available on the Offering Commencement Date of the applicable Offering Period
pursuant to the preceding sentence, notwithstanding any authorization of
additional Shares for issuance under the Plan by the Company's stockholders
subsequent to such Offering Commencement Date.

     12.2. The participant shall have no interest or voting right in Shares
covered by his or her option until such option has been exercised.

     12.3. Shares to be delivered to a participant under the Plan will be
registered in the name of the participant or in the name of the participant and
his or her spouse, as directed by the participant.

13.  ADMINISTRATION

     The Board, or a committee named by the Board, shall supervise and
administer the Plan and shall have full power to adopt, amend and rescind any
rules deemed desirable and appropriate for the administration of the Plan and
not inconsistent with the Plan, to construe and interpret the Plan, and to make
all other determinations necessary or advisable for the administration of the
Plan. The Board's determinations made in good faith on matters referred to in
this Plan shall be final, binding and conclusive on all persons having or
claiming any interest under this Plan.

14.  DESIGNATION OF BENEFICIARY

     14.1. A participant may file a written designation of a beneficiary who is
to receive any Shares and cash, if any, from the participant's account under the
Plan in the event of such participant's death subsequent to the end of an
Offering Period but prior to delivery to him or her of such Shares and cash. Any
such beneficiary shall also be entitled to receive any cash from the
participant's account under the Plan in the event of such participant's death
prior to the Offering Termination Date of an Offering Period.

     14.2. Such designation of beneficiary may be changed by the participant at
any time by written notice. In the event of the death of a participant and in
the absence of a beneficiary validly designated under the Plan who is living at
the time of such participant's death, the Company shall deliver such Shares
and/or cash to the executor or administrator of the estate of the participant,
or if no such executor or administrator has been appointed (to the knowledge of
the Company), the Company, in its discretion, may deliver such Shares and/or
cash to the spouse or to any one or more dependents or relatives of the
participant, or if no spouse, dependent or relative is known to the Company,
then to such other person as the Company may designate.

15.  TRANSFERABILITY OF OPTIONS AND SHARES

     Neither Contributions credited to a participant's account nor any rights
with regard to the exercise of an option or to receive Shares under the Plan may
be assigned, transferred, pledged or

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otherwise disposed of in any way (other than by will, the laws of descent and
distribution, or as provided in Section 14) by the participant. Any such attempt
at assignment, transfer, pledge or other disposition shall be without effect,
except that the Company may treat such act as an election to withdraw funds in
accordance with Section 10. In addition, if the Board has so announced to
Employees at least five (5) days prior to the scheduled beginning of the next
Offering Period to be affected by the Board's determination, any Shares acquired
on the Offering Termination Date of such Offering Period may be subject to
restrictions specified by the Board on the transfer of such Shares. Any
participant selling or transferring any or all of his or her Shares purchased
pursuant to the Plan must provide written notice of such sale or transfer to the
Company within five business days after the date of sale or transfer. Such
notice to the Company shall include the gross sales price, if any, the Offering
Period during which the Shares being sold were purchased by the participant, the
number of Shares being sold or transferred and the date of sale or transfer.

16.  USE OF FUNDS

     All Contributions received or held by the Company under the Plan may be
used by the Company for any corporate purpose, and the Company shall not be
obligated to segregate such Contributions from its other assets.

17.  REPORTS

     Individual accounts will be maintained for each participant in the Plan.
Statements of account will be given to participating Employees at least
annually, which statements will set forth the amounts of Contributions, the per
Share Purchase Price, the number of Shares purchased and the remaining cash
balance, if any.

18.  ADJUSTMENTS UPON CHANGES IN CAPITALIZATION; CORPORATE TRANSACTIONS

     18.1. ADJUSTMENT. Subject to any required action by the stockholders of the
Company, the number of shares covered by each option under the Plan which has
not yet been exercised and the number of Shares which have been authorized for
issuance under the Plan but have not yet been placed under option (collectively,
the "Reserves"), as well as the maximum number of shares of Common Stock which
may be purchased by a participant in an Offering Period, the number of shares of
Common Stock set forth in Section 12.1 above, and the price per Share of Common
Stock covered by each option under the Plan which has not yet been exercised,
shall be proportionately adjusted for any increase or decrease in the number of
the Company's issued Shares resulting from a stock split, reverse stock split,
stock dividend, combination or reclassification of the Common Stock (including
any such change in the number of Shares of Common Stock effected in connection
with a change in domicile of the Company), or any other increase or decrease in
the number of Shares effected without receipt of consideration by the Company;
provided however that conversion of any convertible securities of the Company
shall not be deemed to have been "effected without receipt of consideration."
Such adjustment shall be made by the Board, whose determination in that respect
shall be final, binding and conclusive.

     18.2. CORPORATE TRANSACTIONS. In the event of a dissolution or liquidation
of the Company, the Offering Period then in progress will terminate immediately
prior to the consummation of such action, unless otherwise provided by the
Board. In the event of a Corporate Transaction, each option outstanding under
the Plan shall be assumed or an equivalent option shall be substituted by the
successor corporation or a parent or Subsidiary of such

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successor corporation. In the event that the successor corporation refuses to
assume or substitute for outstanding options, the Offering Period then in
progress shall be shortened and a new Offering Termination Date shall be set
(the "New Offering Termination Date"), as of which date the Offering Period then
in progress will terminate. The New Offering Termination Date shall be on or
before the date of consummation of the transaction and the Board shall notify
each participant in writing, at least ten (10) days prior to the New Offering
Termination Date, that the Offering Termination Date for his or her option has
been changed to the New Offering Termination Date and that his or her option
will be exercised automatically on the New Offering Termination Date, unless
prior to such date he or she has withdrawn from the Offering Period as provided
in Section 10. For purposes of this Section 18, an option granted under the Plan
shall be deemed to be assumed, without limitation, if, at the time of issuance
of the stock or other consideration upon a Corporate Transaction, each holder of
an option under the Plan would be entitled to receive upon exercise of the
option the same number and kind of shares of stock or the same amount of
property, cash or securities as such holder would have been entitled to receive
upon the occurrence of the transaction if the holder had been, immediately prior
to the transaction, the holder of the number of Shares of Common Stock covered
by the option at such time (after giving effect to any adjustments in the number
of Shares covered by the option as provided for in this Section 18); provided
however that if the consideration received in the transaction is not solely
common stock of the successor corporation or its parent (as defined in Section
424(e) of the Code), the Board may, with the consent of the successor
corporation, provide for the consideration to be received upon exercise of the
option to be solely common stock of the successor corporation or its parent
equal in Fair Market Value to the per Share consideration received by holders of
Common Stock in the transaction.

     The Board may, if it so determines in the exercise of its sole discretion,
also make provision for adjusting the Reserves, as well as the price per Share
of Common Stock covered by each outstanding option, in the event that the
Company effects one or more reorganizations, recapitalizations, rights offerings
or other increases or reductions of Shares of its outstanding Common Stock, and
in the event of the Company's being consolidated with or merged into any other
corporation.

19.  AMENDMENT OR TERMINATION

     19.1. The Board may at any time and for any reason terminate or amend the
Plan. Except as provided in Section 18, no termination of the Plan may affect
options previously granted, provided that the Plan or an Offering Period may be
terminated by the Board on an Offering Termination Date or by the Board's
setting a new Offering Termination Date with respect to an Offering Period then
in progress if the Board determines that termination of the Plan and/or the
Offering Period is in the best interests of the Company and its stockholders or
if continuation of the Plan and/or the Offering Period would cause the Company
to incur adverse accounting charges as a result of the Plan. Except as provided
in Section 18 and in this Section 19, no amendment to the Plan shall make any
change in any option previously granted which adversely affects the rights of
any participant.

     19.2. In addition to the foregoing, without stockholder consent and without
regard to whether any participant rights may be considered to have been
adversely affected, the Board (or its committee) shall be entitled to change the
Offering Periods, limit the frequency and/or number of changes in the amount
withheld during an Offering Period, establish the exchange ratio applicable to
amounts withheld in a currency other than U.S. dollars, permit payroll
withholding in excess of the amount designated by a participant in order to
adjust for delays or mistakes in the

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Company's processing of properly completed withholding elections, establish
reasonable waiting and adjustment periods and/or accounting and crediting
procedures to ensure that amounts applied toward the purchase of Common Stock
for each participant properly correspond with amounts withheld from the
participant's Compensation, and establish such other limitations or procedures
as the Board (or its committee) determines in its sole discretion advisable
which are consistent with the Plan.

20.  NOTICES

     Any notice, demand, request or other communication hereunder to any party
shall be deemed to be sufficient if contained in a written instrument delivered
in person or duly sent by first class registered, certified or overnight mail,
postage prepaid, or telecopied with a confirmation copy by regular, certified or
overnight mail, addressed or telecopied, as the ease may be, (i) if to the
recipient of an Award, at his or her residence address last filed with the
Company and (ii) if to the Company, at its principal place of business,
addressed to the attention of its Chief Executive Officer, or to such other
address or telecopier number, as the case may be, as the addressee may have
designated by notice to the addressor. All such notices, requests, demands and
other communications shall be deemed to have been received: (i) in the case of
personal delivery, on the date of such delivery; (ii) in the case of mailing,
when received by the addressee; and (iii) in the case of facsimile transmission,
when confirmed by facsimile machine report connection with the Plan shall be
deemed to have been duly given when received in the form specified by the
Company at the location, or by the person, designated by the Company for the
receipt thereof.

21.  CONDITIONS TO ISSUANCE OF SHARES

     Shares shall not be issued with respect to an option unless the exercise of
such option and the issuance and delivery of such Shares pursuant thereto shall
comply with all applicable provisions of law, domestic or foreign, including,
without limitation, the Securities Act of 1933, as amended, the Exchange Act,
the rules and regulations promulgated thereunder, applicable state securities
laws and the requirements of any stock exchange upon which the Shares may then
be listed, and shall be further subject to the approval of counsel for the
Company with respect to such compliance.

     As a condition to the exercise of an option, the Company may require the
person exercising such option to represent and warrant at the time of any such
exercise that the Shares are being purchased only for investment and without any
present intention to sell or distribute such Shares if, in the opinion of
counsel for the Company, such a representation is required by any of the
aforementioned applicable provisions of law.

22.  TERM OF PLAN; EFFECTIVE DATE

     The Plan shall become effective upon the IPO Date. It shall continue in
effect for a term of five (5) years unless sooner terminated under Section 19.

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                                                  FORM OF SUBSCRIPTION AGREEMENT

                               ACTIVBIOTICS, INC.

                        2006 EMPLOYEE STOCK PURCHASE PLAN

                             SUBSCRIPTION AGREEMENT

New Election: ___

Change of Election: ___

     1. I, ________________, hereby elect to participate in the ActivBiotics,
Inc. 2006 Employee Stock Purchase Plan (as amended, the "Plan") for the Offering
Period _____________ ___, _____ to ___________ _____, _____, and subscribe to
purchase shares of the Company's Common Stock in accordance with this
Subscription Agreement and the Plan.

     2. I elect to have Contributions in the amount of ___% of my Compensation,
as those terms are defined in the Plan, applied to this purchase. I understand
that this amount must not be less than 1% and not more than 15% of my
Compensation during the Offering Period. (Please note that no fractional
percentages are permitted).

     3. I hereby authorize payroll deductions from each paycheck during the
Offering Period at the rate stated in Item 2 of this Subscription Agreement. I
understand that all payroll deductions made by me shall be credited to my
account under the Plan and that I may not make any additional payments into such
account. I understand that all payments made by me shall be accumulated, without
interest or earnings, for the purchase of shares of Common Stock at the
applicable purchase price determined in accordance with the Plan. I further
understand that, except as otherwise set forth in the Plan, shares will be
purchased for me automatically on the Offering Termination Date of each Offering
Period unless I otherwise withdraw from the Plan by giving written notice to the
Company for such purpose.

     4. I understand that I may discontinue at any time prior to the Offering
Termination Date my participation in the Plan as provided in Section 10 of the
Plan. I acknowledge that, unless I discontinue my participation in the Plan as
provided in Section 10 of the Plan, my election will continue to be effective
for each successive Offering Period.

     5. I have received a copy of the complete ActivBiotics, Inc. 2006 Employee
Stock Purchase Plan. I understand that my participation in the Plan is in all
respects subject to the terms of the Plan.

     6. Shares purchased for me under the Plan should be issued in the name(s)
of (name of employee or employee and spouse only):

                     _____________________________________

                     _____________________________________

     7. In the event of my death, I hereby designate the following as my
beneficiary(ies) to receive all payments and shares due to me under the Plan:

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NAME: (Please print)                    ________________________________________
                                        (First)         (Middle)          (Last)


_____________________________________   ________________________________________
(Relationship)                          (Address)

                                        ________________________________________

     8. I understand that if I dispose of any shares received by me pursuant to
the Plan within 2 years after the Offering Commencement Date (the first day of
the Offering Period during which I purchased such shares) or within 1 year after
the Offering Termination Date, I will be treated for federal income tax purposes
as having received ordinary compensation income at the time of such disposition
in an amount equal to the excess of the fair market value of the shares on the
Offering Termination Date over the price which I paid for the shares, regardless
of whether I disposed of the shares at a price less than their fair market value
at the Offering Termination Date. The remainder of the gain or loss, if any,
recognized on such disposition will be treated as capital gain or loss.

     I hereby agree to notify the Company in writing within 30 days after the
date of any such disposition, and I will make adequate provision for federal,
state or other tax withholding obligations, if any, which arise upon the
disposition of the Common Stock. The Company may, but will not be obligated to,
withhold from my compensation the amount necessary to meet any applicable
withholding obligation including any withholding necessary to make available to
the Company any tax deductions or benefits attributable to the sale or early
disposition of Common Stock by me.

     9. If I dispose of such shares at any time after expiration of the 2-year
and 1-year holding periods, I understand that I will be treated for federal
income tax purposes as having received compensation income only to the extent of
an amount equal to the lesser of (1) the excess of the fair market value of the
shares at the time of such disposition over the purchase price which I paid for
the shares under the option, or (2) 15% of the fair market value of the shares
on the Offering Commencement Date. The remainder of the gain or loss, if any,
recognized on such disposition will be treated as capital gain or loss.

     I understand that this tax summary is only a summary and is subject to
change. I further understand that I should consult a tax advisor concerning
certain tax implications of the purchase and sale of stock under the Plan.

     10. I hereby agree to be bound by the terms of the Plan. The effectiveness
of this Subscription Agreement is dependent upon my eligibility to participate
in the Plan.


                                        SIGNATURE:
                                                   -----------------------------

                                        SOCIAL SECURITY #:
                                                           ---------------------

                                        DATE:
                                              ----------------------------------

<Page>

                                                    FORM OF NOTICE OF WITHDRAWAL

                               ACTIVBIOTICS, INC.

                        2006 EMPLOYEE STOCK PURCHASE PLAN

                              NOTICE OF WITHDRAWAL

     I, ____________, hereby elect to withdraw my participation in the
ActivBiotics, Inc. 2006 Employee Stock Purchase Plan (the "Plan") for the
Offering Period that began on ________________, _________. This withdrawal
covers all Contributions credited to my account and is effective on the date
designated below.

     I understand that all Contributions credited to my account will be paid to
me within ten (10) business days of receipt by the Company of this Notice of
Withdrawal and that my option for the current period will automatically
terminate, and that no further Contributions for the purchase of shares can be
made by me during the Offering Period.

     The undersigned further understands and agrees that be or she shall be
eligible to participate in succeeding Offering Periods only by delivering to the
Company a new Subscription Agreement.


Dated:
       ------------------------------   ----------------------------------------
                                                  Signature of Employee


                                        ----------------------------------------
                                                 Social Security Number