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                                                                     Exhibit 4.2

               FOURTH AMENDED AND RESTATED STOCKHOLDERS' AGREEMENT

     This FOURTH AMENDED AND RESTATED STOCKHOLDERS' AGREEMENT (the "Agreement"),
dated this 30th day of December, 2005, is entered into by and among (i)
ACTIVBIOTICS, INC., a Delaware corporation (the "Corporation"), (ii) those
holders of Common Stock (as defined below) listed on SCHEDULE 1 hereto
(hereinafter referred to collectively as the "Original Stockholders"), (iii)
those holders of Series A Preferred Stock (as defined below) listed on SCHEDULE
2 hereto (hereinafter referred to collectively as the "Series A Stockholders"),
(iv) those holders of the Series B Preferred Stock (as defined below) listed on
SCHEDULE 3 hereto (hereinafter referred to collectively as the "Series B
Stockholders"), (v) those holders of the Series B-1 Preferred Stock (as defined
below) listed on SCHEDULE 4 hereto (hereinafter referred to collectively as the
"Series B-1 Stockholders"), (vi) those holders of the Series C Preferred Stock
(as defined below) listed on SCHEDULE 5 hereto (hereinafter referred to
collectively as the "Series C Stockholders"), (vii) those holders of the Series
C-2 Preferred Stock (as defined below) who become parties hereto by executing
and delivering a Letter of Transmittal (as defined below) in accordance with the
applicable provisions of the Merger Agreement (as defined below), and are listed
on SCHEDULE 6 hereto, as such schedule may be amended from time to time as set
forth in Section 14 hereof (hereinafter referred to collectively as the "Series
C-2 Stockholders" and together with the Series A Stockholders, the Series B
Stockholders, the Series B-1 Stockholders, the Series C Stockholders and the
Series C-1 Stockholders (as defined below), the "Investors"), and (viii) those
holders of shares of capital stock of ActivBiotics (Canada) Inc., a Canadian
corporation ("ABI Canada"), listed on SCHEDULE 7 hereto (hereinafter referred to
collectively as the "Canadian Investors"). This Agreement amends and restates
and replaces in its entirety that certain Third Amended and Restated
Stockholders' Agreement, dated March 22, 2005, by and among Corporation and the
holders of capital stock of the Corporation that are parties thereto (the
"Original Agreement").

                                   WITNESSETH:

     WHEREAS, the Corporation, Metaphore Acquisition Co., a Delaware corporation
and wholly-owned subsidiary of the Corporation ("Merger Sub"), Metaphore
Pharmaceuticals, Inc., a Delaware corporation ("Metaphore"), and the Stockholder
Representatives party thereto entered into an Agreement and Plan of Merger and
Reorganization, dated December 7, 2005 (the "Merger Agreement"), pursuant to
which Merger Sub merged with and into Metaphore with Metaphore continuing as the
surviving corporation and, in connection therewith (and on the terms and
conditions set forth therein), the Corporation agreed, among other things, to
issue to the stockholders of Metaphore shares of the Corporation's Series C-2
Convertible Preferred Stock, par value $0.01 per share (the "Series C-2
Preferred Stock");

     WHEREAS, it is a condition precedent to the Merger Agreement that the
Original Agreement be amended and restated to read in its entirety as set forth
in this Agreement;

     WHEREAS, in accordance with Section 13 of the Original Agreement, the
Original Agreement may not be amended except pursuant to the written consent of
the Corporation and the holders of a majority of the Preferred Shares (as
defined in the Original Agreement), voting together as a single class on an
as-converted basis; and

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     WHEREAS, in connection with the transactions contemplated by the Merger
Agreement, the Corporation and those of the Series A Stockholders, the Series B
Stockholders, the Series B-1 Stockholders, the Series C Stockholders, the
Original Stockholders and the Canadian Investors whose signatures are set forth
on the signature pages hereto desire, for and on behalf of all of the parties to
the Original Agreement, to amend and restate in its entirety the Original
Agreement on the terms and conditions set forth herein, and the Preferred
Stockholders whose signatures are set forth on the signature pages hereto
represent holders of no less than a majority of the Preferred Shares (as defined
in the Original Agreement), voting together as a single class on an as-converted
basis.

     NOW, THEREFORE, in consideration of the foregoing and for other good and
valuable consideration, the receipt of which is hereby acknowledged, the parties
hereto hereby agree as follows:

     SECTION 1. Definitions. As used herein, the following terms shall have the
following respective meanings:

     20-Day Period shall have the meaning set forth in Section 3.3(b) hereof.

     ABI Canada shall have the meaning set forth in the preamble hereto.

     B Director shall have the meaning set forth in Section 6.1(c) hereof.

     B Preferred Shares shall mean, collectively, the Series B Preferred Shares
and the Series B-1 Preferred Shares.

     B Preferred Stock shall mean, collectively, the Series B Preferred Stock
and the Series B-1 Preferred Stock.

     B Stockholders shall mean, collectively, the Series B Stockholders and the
Series B-1 Stockholders.

     BioVentures Director shall have the meaning set forth in Section 6.1(e)
hereof.

     BioVentures Investors shall have the meaning set forth in Section 6.1(e)
hereof.

     Board or Board of Directors shall mean the Board of Directors of the
Corporation.

     Budget shall have the meaning set forth in Section 3.8 hereof.

     Canadian Financing shall mean a financing of up to US $10,000,000 through
ABI Canada.

     Canadian Investors shall have the meaning (severally, but not jointly) set
forth in the preamble hereto.

     CMDF shall mean Canadian Medical Discoveries Fund Inc.


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     Certificate shall mean the Sixth Amended and Restated Certificate of
Incorporation of the Corporation, as amended and in effect from time to time.

     Commission shall mean the U.S. Securities and Exchange Commission.

     Common Stock shall mean the Common Stock, par value $0.01 per share, of the
Corporation.

     Designating Party shall have the meaning set forth in Section 6.5 hereof.

     Environmental Laws shall mean all applicable federal, state and local laws,
ordinances, rules and regulations that regulate, fix liability for, or otherwise
relate to, the handling, use (including use in industrial processes, in
construction, as building materials, or otherwise), storage and disposal of
hazardous and toxic wastes and substances, and to the discharge, leakage,
presence, migration, threatened release or release (whether by disposal, a
discharge into any water source or system or into the air, or otherwise) of any
pollutant or effluent. Without limiting the preceding sentence, the term
"Environmental Laws" shall specifically include the following federal and state
laws, as amended:

FEDERAL

          Comprehensive Environmental Response, Compensation and Liability Act
          of 1980, 42 U.S.C. 9601 et. seq.;

          Resource Conservation and Recovery Act of 1976, 42 U.S.C. 6901 et.
          seq.;

          Federal Water Pollution Control Act, 33 U.S.C. 1251 et. seq.; and

          Clean Air Act, 42 U.S.C. 7401 et. seq.

STATE

MASSACHUSETTS ENVIRONMENTAL STATUTES

          Massachusetts Clean Waters Act, Mass. Gen. L. Ch. 21, Section 26, et.
          seq., and regulations thereto;

          Massachusetts Solid Waste Disposal Laws, Mass. Gen. L. Ch. 16, Section
          18, et. seq., and Ch. 111, Section 105A, and regulations thereto;

          Massachusetts Oil and Hazardous Materials Release Prevention and
          Response Act, Mass. Gen. L., Ch. 21E, Section 1, et. seq., and
          regulations thereto;


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          Massachusetts Solid Waste Facilities Law, Mass. Gen. L., Ch. 21H,
          Section 1, et. seq., and regulations thereto;

          Massachusetts Toxic Use Reduction Act, Mass. Gen. L., Ch. 21I, Section
          1, et. seq., and regulations thereto;

          Massachusetts Litter Control Laws, Mass. Gen. L. Ch. 111, Section
          150A, et. seq., and regulations thereto;

          Massachusetts Wetlands Protection Laws, Mass. Gen. L., Ch. 130,
          Section 105, et. seq., and regulations thereto;

          Massachusetts Environmental Air Pollution Control Law, Mass. Gen. L.,
          Ch. 101, Section 2B, et. seq., and regulations thereto;

          Massachusetts Environmental Policy Act, Mass. Gen. L. 30, Section 61,
          et. seq., and regulations thereto; and

          Massachusetts Hazardous Waste Laws, Mass. Gen. L. Ch. 21C, Section 1,
          et. seq., and regulations thereto.

     Equity Percentage shall mean, as to any Investor and any Canadian Investor,
that percentage figure which expresses the ratio that (a) the number of shares
of issued and outstanding Common Stock then owned by such Investor and Canadian
Investor or attributable to such Canadian Investor bears to (b) the aggregate
number of shares of issued and outstanding Common Stock then owned by all
Investors and Canadian Investors or attributable to such Canadian Investors. For
purposes solely of the computations set forth in clauses (a) and (b) above, the
Common Stock attributable to any Canadian Investor shall be the shares of Common
Stock issuable upon conversion of the shares of Series C-1 Preferred Stock that
would be issued pursuant to the Put and Support Agreement (including shares of
Common Stock issued or issuable upon conversion thereof, or shares of Common
Stock issued as if shares of Series C-1 Preferred Stock had so converted) as if
such shares of Series C-1 Preferred Stock had been issued to a Canadian Investor
upon the exercise of the Put Right (as defined in the Put and Support Agreement)
pursuant to the terms and conditions of the Put and Support Agreement. For
purposes solely of the computation set forth in clauses (a) and (b) above and
the right of oversubscription, all issued and outstanding securities held by the
Investors and Canadian Investors that are convertible into or exercisable or
exchangeable for shares of Common Stock (including any issued and outstanding
shares of Preferred Stock) or for any such convertible, exercisable or
exchangeable securities, shall be treated as having been so converted, exercised
or exchanged for such Common Stock or for such convertible, exercisable or
exchangeable securities (which shall be treated as having been further
converted, exercised or exchanged for such Common Stock) all at the rate(s) or
price(s) at which such securities are convertible, exercisable or exchangeable
(and, as applicable, further convertible, exercisable or exchangeable) for
shares of Common Stock in effect at the time in question (which, for purposes of
Section 3.3 hereof, shall be at the time of delivery by the Corporation of the
Offer Notice contemplated by Section 3.3(b)), whether or not such securities are
at such time immediately


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convertible, exercisable or exchangeable or, as applicable, further convertible,
exercisable or exchangeable.

     Excess Securities shall have the meaning set forth in Section 3.3(d)
hereof.

     Excess Securities Notice shall have the meaning set forth in Section 3.3(d)
hereof.

     Excess Securities Period shall have the meaning set forth in Section 3.3(d)
hereof.

     Exchange Act shall mean the Securities Exchange Act of 1934, as amended.

     Exchange Act Registration Statement shall have the meaning set forth in
Section 3.5 hereof.

     Excluded Forms shall have the meaning given such term in Section 4.5(a)
hereof.

     Excluded Securities shall have the meaning herein as the term "Excluded
Stock" is defined to have in Article III, Section A.7(d)(ii) of the Certificate.

     Group shall mean (i) as to an Investor that is a corporation, any and all
of the venture capital limited partnerships or corporations now existing or
hereafter formed that are affiliated with or under common control with one or
more of the controlling stockholders of such Investor and any predecessor or
successor thereto, (ii) in the case of HCV VI, the HCV Group, (iii) in the case
of MDS, the MDS Group, (iv) in the case of VenGrowth, the VenGrowth Group, (v)
as to any Investor, any other Investor, and (vi) as to any Canadian Investor,
the other Canadian Investor.

     Hazardous Materials shall include without limitation, any flammable
explosives, petroleum products, petroleum byproducts, radioactive materials,
hazardous wastes, hazardous substances, toxic substances or other similar
materials regulated by Environmental Laws.

     HCV Director shall have the meaning set forth in Section 6.1(b) hereof.

     HCV Group shall mean, (i) HCV VI, (ii) any venture capital limited
partnership now existing or hereafter formed which is affiliated with or under
common control with one or more general partners of any general partner of HCV
VI (an "HCV Fund"), (iii) any limited partners or affiliates of HCV VI or any
other HCV Fund and (iv) any successors or assigns of any of the foregoing.

     HCV VI shall mean HealthCare Ventures VI, L.P., a Delaware limited
partnership, including any successor thereto or any assignee of the interest, in
whole or in part, of HCV IV under this Agreement.

     Investors shall have the meaning (severally, but not jointly) set forth in
the preamble hereto.


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     Letter of Transmittal shall mean shall mean a Letter of Transmittal
substantially in the form attached to the Merger Agreement as Exhibit H thereto.

     Majority Series C-2 Holders shall have the meaning set forth in Section
6.1(d) hereof.

     Merger Agreement shall have the meaning set forth in the preamble hereto.

     Metaphore shall have the meaning set forth in the preamble hereto.

     MDS shall mean MDS Capital Corp.

     MDS Group shall mean (i) MDS, (ii) CMDF, (iii) any corporation, trust,
partnership, limited liability corporation, partnership or other form of
business entity which is an investment fund to which MDS or any of its
affiliates provides investment management and/or advisory services (which for
greater certainty currently includes, among others, MDS Life Sciences Technology
Fund II NC Limited Partnership, MDS Life Sciences Technology Fund II Quebec
Limited Partnership, MLII Co-Investment Fund NC Limited Partnership and SC
Biotechnology Development Fund LP) (an "MDS Fund"), (iv) any limited partners or
affiliates of MDS or any MDS Fund, (v) any venture capital limited partnership
now existing or hereafter formed which is affiliated with or under common
control with one or more general partners of any general partner of MDS and (vi)
any successors or assigns of any of the foregoing.

     Notice of Acceptance shall have the meaning set forth in Section 3.3(c)
hereof.

     Offer shall have the meaning set forth in Section 3.3(a) hereof.

     Offered Securities shall mean (i) any shares of Common Stock, Preferred
Stock or any other equity security of the Corporation, (ii) any debt security or
capitalized lease with any equity feature with respect to the Corporation, or
(iii) any option, warrant or other right to subscribe for, purchase or otherwise
acquire any such equity security, debt security or capitalized lease; provided
however, Offered Securities shall not include (x) the Excluded Securities and(y)
securities issued and outstanding as of the date of this Agreement.

     Original Agreement shall have the meaning set forth in the preamble hereto.

     Other Senior C Directors shall have the meaning set forth in Section 6.1(d)
hereof.

     Other Shares shall have the meaning set forth in Section 4.5(e) hereof.

     Preferred Shares shall mean the Series A Preferred Shares, the Series B
Preferred Shares, the Series B-1 Preferred Shares, the Series C Preferred
Shares, the Series C-1 Preferred Shares and the Series C-2 Preferred Shares.

     Preferred Stock shall mean, collectively, the Series A Preferred Stock, the
Series B Preferred Stock, the Series B-1 Preferred Stock, the Series C Preferred
Stock, the Series C-1 Preferred Stock and the Series C-2 Preferred Stock.


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     Preferred Stockholders shall mean, collectively, the Series A Stockholders,
the Series B Stockholders, the Series B-1 Stockholders, the Series C
Stockholders, the Series C-1 Stockholders and the Series C-2 Stockholders.

     Property shall include, without limitation, land, buildings and laboratory
facilities owned or leased by the Corporation or as to which the Corporation now
has any duties, responsibilities (for clean-up, remedy or otherwise) or
liabilities under any Environmental Laws, or as to which the Corporation or any
subsidiary of the Corporation may have such duties, responsibilities or
liabilities because of past acts or omissions of the Corporation or any such
subsidiary or their predecessors, or because the Corporation or any such
subsidiary or their predecessors in the past was such an owner or operator of,
or bore some other relationship with, such land, buildings and/or laboratory
facilities.

     Put and Support Agreement shall mean that certain Put and Support
Agreement, dated March 22, 2005, by and among the Corporation, ABI Canada and
the Canadian Investors, as amended and in effect from time to time.

     Put Right shall have the meaning set forth in the Put and Support
Agreement.

     Qualified Public Offering shall have the meaning set forth in Section 3.17
hereof.

     Refused Securities shall have the meaning set forth in Section 3.3(f)
hereof.

     Restricted Securities shall mean any of the Preferred Shares and the Common
Stock issued or issuable upon the conversion of the Preferred Shares, all shares
of Common Stock issued or issuable in respect thereof by way of stock splits,
stock dividends, stock combinations, recapitalizations or like occurrences, and
any other shares of Common Stock or other securities of the Corporation which
may be issued hereafter to any of the Investors or any Canadian Investor or any
member of their Group (including without limitation any shares of Series C-1
Preferred Stock issuable pursuant to the exercise of the Put Right under the Put
and Support Agreement) which are convertible into or exercisable for shares of
Common Stock (including, without limitation, other classes or series of
Preferred Stock, warrants, options or other rights to purchase Common Stock or
convertible debentures or other convertible debt securities) and the Common
Stock issued or issuable upon such conversion or exercise of such other
securities, which have not been sold (a) pursuant to an effective registration
statement filed pursuant to the Securities Act, or (b) pursuant to Rule 144 or
Rule 144A promulgated by the Commission under the Securities Act.

     Restricted Shares shall mean the shares of Common Stock issued or issuable
upon the conversion or exchange of the Restricted Securities or otherwise
constituting a portion of the Restricted Securities.

     Securities Act shall mean the Securities Act of 1933, as amended.

     Senior C Directors shall have the meaning set forth in Section 6.1(d)
hereof.


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     Senior C Preferred Shares shall mean, collectively, the Series C Preferred
Shares, the Series C-1 Preferred Shares and the Series C-2 Preferred Shares.

     Senior C Preferred Stock shall mean, collectively, the Series C Preferred
Stock, the Series C-1 Preferred Stock and the Series C-2 Preferred Stock.

     Senior C Stockholders shall mean, collectively, the Series C Stockholders,
the Series C-1 Stockholders and the Series C-2 Stockholders.

     Series A Directors shall have the meaning set forth in Section 6.1(b)
hereof.

     Series A Preferred Shares shall mean issued and outstanding shares of the
Series A Preferred Stock.

     Series A Preferred Stock shall mean Series A Convertible Preferred Stock,
par value $.01 per share, of the Corporation.

     Series A Stockholders shall have the meaning set forth in the preamble
hereto.

     Series B Preferred Shares shall mean issued and outstanding shares of
Series B Preferred Stock.

     Series B Preferred Stock shall mean Series B Convertible Preferred Stock,
par value $.01 per share, of the Corporation.

     Series B Stockholders shall have the meaning set forth in the preamble
hereto.

     Series B-1 Preferred Shares shall mean issued and outstanding shares of
Series B-1 Preferred Stock.

     Series B-1 Preferred Stock shall mean Series B-1 Convertible Preferred
Stock, par value $.01 per share, of the Corporation.

     Series B-1 Stockholders shall have the meaning set forth in the preamble
hereto.

     Series C Preferred Shares shall mean issued and outstanding shares of
Series C Preferred Stock.

     Series C Preferred Stock shall mean Series C Convertible Preferred Stock,
par value $.01 per share, of the Corporation.

     Series C Stock Purchase Agreement shall mean the Series C Convertible
Preferred Stock Purchase Agreement, dated as of December 22, 2004, by and among
the Corporation and the purchasers of Series C Preferred Stock parties thereto,
as amended and in effect from time to time.

     Series C Stockholders shall have the meaning set forth in the preamble
hereto.


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     Series C-1 Preferred Shares shall mean issued and outstanding shares of
Series C-1 Preferred Stock.

     Series C-1 Preferred Stock shall mean Series C-1 Convertible Preferred
Stock, par value $.01 per share, of the Corporation.

     Series C-1 Stockholders shall mean those holders of shares of Series C-1
Preferred Stock who acquired such shares of Series C-1 Preferred Stock pursuant
to the Put and Support Agreement, and are listed on SCHEDULE 8 hereto, as such
schedule may be amended from time to time as set forth in Section 14 hereof.

     Series C-2 Preferred Shares shall mean issued and outstanding shares of
Series C-2 Preferred Stock..

     Series C-2 Preferred Stock shall mean Series C-2 Convertible Preferred
Stock, par value $.01 per share, of the Corporation.

     Series C-2 Stockholders shall have the meaning set forth in the preamble
hereto.

     Stockholders shall mean all holders of capital stock of the Corporation.

     Transfer shall mean any disposition of any Restricted Securities or of any
interest therein which would constitute a sale thereof within the meaning of the
Securities Act.

     Unaffiliated HCV Directors shall have the meaning set forth in Section
6.1(b) hereof.

     VenGrowth shall mean The VenGrowth Advanced Life Sciences Fund Inc.

     VenGrowth Director shall have the meaning set forth in Section 6.1(d)
hereof.

     VenGrowth Group shall mean (i) VenGrowth, (ii) VenGrowth Private Equity
Partners Inc. ("VenGrowth Partners"), (iii) any corporation, trust partnership,
limited liability corporation, partnership or other form of business entity
which is an investment fund to which VenGrowth or VenGrowth Partners or any of
its affiliates provides investment management and/or advisory services (which
for greater certainty currently includes, among others, (a) any limited partners
or affiliates of VenGrowth or VenGrowth Partners, (b) any venture capital
limited partnership now existing or hereafter formed which is affiliated with or
under common control with one or more general partners of any general partner of
VenGrowth or VenGrowth Partners) and (iv) any successors or assigns of any of
the foregoing.

     SECTION 2. CMDF and VenGrowth Rights and Obligations.

          2.1. CMDF and VenGrowth Rights and Obligations. The Corporation shall
not, and the parties hereto agree that the Corporation shall not, take any
action in respect of which the prior consent, approval or request of those
persons holding a specified percentage or proportion of shares of Common Stock
and/or any or all series of Preferred Stock issued and outstanding or a
specified percentage or proportion of the voting power of shares of Common


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Stock and/or any or all series of Preferred Stock issued and outstanding (in
either case whether the shares of Common Stock and/or any or all series of
Preferred Stock are voting, consenting, approving or requesting together as a
single class or are voting, consenting, approving or requesting separately by
class or series), is required (i) by the Certificate, (ii) under this Agreement,
(iii) under the Series C Stock Purchase Agreement, or (iv) under any applicable
law, unless, in each case, (x) the Corporation provides each Canadian Investor
with not less than ten (10) days prior notice with respect to the consent,
approval or request sought or the action to be taken, and (y) such consent,
approval or request is given by persons (including, without limitation, Canadian
Investors, if required) holding shares of Preferred Stock and/or Common Stock
issued and outstanding (after giving effect to the provisions the next sentence)
at the time such consent, approval or request is sought that represent such
specified percentage or proportion. Solely for purposes of this Section 2, all
of the preferred shares of ABI Canada that are outstanding shall be deemed to
have been put for shares of Preferred Stock or Common Stock, as the case may be,
pursuant to the Put and Support Agreement, and such shares of Preferred Stock or
Common Stock shall be deemed to be issued and outstanding, immediately prior to
the effectiveness of any consent, approval or request subject to this Section
2.1.

          2.2. Excluded Rights. For greater certainty, and without limiting the
generality of the foregoing, no Canadian Investor may exercise any voting or
other rights in the Corporation in respect of voting to elect the directors of
the Corporation pursuant to Section 6 hereof or the Certificate unless and until
such Canadian Investor, upon the exercise of the Put Right pursuant to the terms
and conditions of the Put and Support Agreement, becomes a Series C-1
Stockholder. At such time that a Canadian Investor becomes a Series C-1
Stockholder, all restrictions on voting or other rights in the Corporation in
respect of voting set forth in the first sentence of this Section 2.2 shall
terminate immediately with respect to such Canadian Investor, and such Canadian
Investor shall agree to be bound by the terms and provisions of Section 6
hereof.

          2.3 Duration of Section. Sections 2.1 and 2.2 hereof and the rights
and obligations of the parties thereunder shall automatically terminate on the
earlier of (A) the consummation of (i) a Qualified Public Offering (as defined
in Section 3.17 hereof) or (ii) an Approved Sale (as defined in Section 3.14
hereof) and (B) the date on which all Canadian Investors shall have become
Series C-1 Stockholders.

     SECTION 3. Certain Covenants of the Corporation.

          3.1. Meetings of the Board of Directors. The Corporation shall call,
and use its best efforts to have, regular meetings of the Board not less often
than quarterly. The Corporation shall pay all reasonable and appropriately
documented travel expenses and other out-of-pocket expenses incurred by
directors who are not employed by the Corporation in connection with attendance
at meetings to transact the business of the Corporation or attendance at
meetings of the Board or any committee thereof.

          3.2. Reservation of Shares of Common Stock and Preferred Stock, Etc.
The Corporation shall at all times have authorized and reserved out of its
authorized but unissued shares of Common Stock, a sufficient number of shares of
Common Stock to provide for the


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conversion of the Preferred Shares. Neither the issuance of the Preferred Shares
to the Investors nor the shares of Common Stock issuable upon the conversion of
such Preferred Shares shall be subject to any preemptive right of any
Stockholder.

          3.3. Right of First Refusal.

          (a) The Corporation shall not issue, sell or exchange, agree to issue,
sell or exchange, or reserve or set aside for issuance, sale or exchange, any
Offered Securities unless in each case the Corporation shall have first offered
to sell to the Investors and the Canadian Investors all of such Offered
Securities on the terms set forth herein (the "Offer"). Each Investor and
Canadian Investors shall be entitled to purchase up to its Equity Percentage of
the Offered Securities. In addition, each Investor and Canadian Investor shall
have a right of oversubscription ("Right of Oversubscription") such that if any
Investor or Canadian Investor fails to accept the Offer as to its Equity
Percentage of the Offered Securities, the remaining Investors and Canadian
Investor shall, among them, have the right to purchase up to the balance of the
Offered Securities not so purchased. Such Right of Oversubscription may be
exercised by an Investor or a Canadian Investor (each, an "Oversubscribing
Investor") by accepting the Offer as to more than its Equity Percentage of the
Offered Securities. If as a result thereof, such oversubscriptions exceed the
total number of the Offered Securities available in respect of such
oversubscription privilege, the Oversubscribing Investors shall be cut back with
respect to their oversubscriptions on a pro rata basis. Each Investor and
Canadian Investor may delegate or assign its rights and obligations with respect
to such Offer to one or more members of its Group, which members shall
thereafter be deemed to be "Investors" or "Canadian Investors", as applicable,
for the purpose of applying this Section 3.3 to such Offer. Each Canadian
Investor, to the extent that it does not exercise its rights under this Section
3.3, shall have the option to receive the right to subscribe for an equivalent
number of shares in ABI Canada; provided, that a number of shares of ABI Canada
are issued to the Corporation simultaneously so as to maintain equal ownership
percentage of ABI Canada with the Canadian Investors (i.e., the Corporation owns
50% and the Canadian Investors own 50% at all times) and the provisions of this
Agreement shall apply mutatis mutandis and the parties shall agree to make any
conforming changes to the Put and Support Agreement to reflect the right to
purchase additional shares in ABI Canada (pursuant to this paragraph). Such
additional shares of ABI Canada will be subject to the put provisions contained
in the Put and Support Agreement.

          (b) The Corporation shall deliver to each Investor and Canadian
Investor written notice of the Offer, specifying the price and terms and
conditions of the Offer (the "Offer Notice"). The Offer by its terms shall
remain open and irrevocable for a period of 20 days from the date of delivery of
the Offer Notice to each Investor and Canadian Investor (the "20-Day Period"),
subject to extension to include the Excess Securities Period (as such term is
hereinafter defined).

          (c) Each Investor and Canadian Investor shall evidence its intention
to accept the Offer by delivering a written notice signed by the Investor or
Canadian Investor, as applicable, setting forth the number of shares that the
Investor or Canadian Investor elects to purchase (the "Notice of Acceptance").
The Notice of Acceptance must be delivered to the Corporation not later than the
last day of the 20-Day Period.


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          (d) If any Investor or Canadian Investor fails to exercise its rights
hereunder to purchase all or any portion of its Equity Percentage of the Offered
Securities, the Corporation shall so notify the other Investors and Canadian
Investors in a written notice (the "Excess Securities Notice"). The Excess
Securities Notice shall be given by the Corporation promptly after it learns of
any Investor's or Canadian Investor's intention not to purchase all or any
portion of its Equity Percentage of the Offered Securities, but in no event
later than ten (10) days after the expiration of the 20-Day Period. The
Investors and Canadian Investors who or which have agreed to purchase their
Equity Percentage of the Offered Securities shall have the right to purchase the
portion not purchased by such other Investor(s) or Canadian Investor(s) (the
"Excess Securities"), on a pro rata basis, by giving notice within ten (10) days
after receipt of the Excess Securities Notice from the Corporation. The twenty
(20) day period during which (i) the Corporation must give the Excess Securities
Notice to the other Investors and Canadian Investors, and (ii) each of the other
Investors and Canadian Investors must give the Corporation notice of its
intention to purchase all or any portion of its pro rata share of its Excess
Securities is hereinafter referred to as the "Excess Securities Period."

          (e) If the Investors or Canadian Investors tender their Notices of
Acceptance prior to the end of the 20-Day Period indicating their intention to
purchase all of the Offered Securities, or if prior to the termination of the
Excess Securities Period, the Investors or Canadian Investors tender Excess
Securities Notices to purchase all of the Excess Securities, the Corporation
shall proceed to promptly schedule a closing of the sale of all such Offered
Securities at which closing each Investor and Canadian Investor shall (i)
purchase from the Corporation that portion of the Offered Securities (including
the Excess Securities) for which it tendered a Notice of Acceptance and an
Excess Securities Notice, if applicable, upon the terms specified in the Offer
Notice, and (ii) execute and deliver an agreement further restricting transfer
of such Offered Securities substantially as set forth in Sections 4.1, 4.2 and
4.3 hereof. In addition, with respect to the Offered Securities being purchased
by the Investors and Canadian Investors at such closing, the Corporation shall
provide each such Investor and Canadian Investor with the rights and benefits
set forth in this Agreement. The obligation of each Investor and Canadian
Investor to purchase its respective portion of such Offered Securities at such
closing is further conditioned upon the preparation of a purchase agreement
embodying the terms of the Offer, which shall be reasonably satisfactory in form
and substance to such Investor and such Investor's counsel and to such Canadian
Investor and such Canadian Investor's counsel.

          (f) In the event the Corporation does not sell all of the Offered
Securities pursuant to Section 3.3(e) hereof, the Corporation shall have ninety
(90) days from the expiration of 20 Day Period or, if applicable, the Excess
Securities Period to sell the portion of the Offered Securities (including the
Excess Securities) refused by the Investors and Canadian Investors (the "Refused
Securities") to any other person or persons, but only upon terms and conditions
which are in all material respects (including, without limitation, price and
interest rate) no more favorable to such other person or persons, and no less
favorable to the Corporation, than those set forth in the Offer Notice. Upon and
subject to the closing of the sale of all of the Refused Securities (which shall
include full payment to the Corporation), each Investor and Canadian Investor
shall (i) purchase from the Corporation those Offered Securities (including the
Excess Securities) for which it tendered a Notice of Acceptance or an Excess
Securities Notice, as applicable, upon the terms specified in the Offer Notice,
and (ii) execute and deliver an


                                       12

<Page>

agreement restricting transfer of such Offered Securities (including the Excess
Securities) substantially as set forth in Sections 4.1, 4.2 and 4.3 hereof. In
addition, with respect to the Offered Securities being purchased by the
Investors and Canadian Investors, the Corporation shall provide each such
Investor and Canadian Investor with the rights and benefits set forth in this
Agreement. The Corporation agrees, as a condition precedent to accepting payment
for and making delivery of any Refused Securities to any person other than an
Investor or Canadian Investor, to have each and every such person execute and
deliver a Stock Restriction Agreement in the capacity of a "Principal
Stockholder" as defined therein, substantially in the form attached hereto as
Exhibit A, or as may be modified or amended from time to time with the prior
approval of the holders of a majority of the combined voting power of the
Preferred Shares then outstanding, calculated in accordance with Section A.6(a)
of Article III of the Certificate (but including in such calculation, any
outstanding Restricted Shares held by such holders), to the extent such
purchaser has not already executed such Agreement. The obligation of any
Investor or Canadian Investor to purchase any portion of such Offered Securities
(including the Excess Securities) is further conditioned upon the preparation of
a purchase agreement embodying the terms of the Offer, which shall be reasonably
satisfactory in form and substance to such Investor and such Investor's counsel
and to such Canadian Investor and such Canadian Investor's counsel.

          (g) In each case, any Offered Securities not purchased either by the
Investors or Canadian Investors or by any other person in accordance with this
Section 3.3 may not be sold or otherwise disposed of until they are again
offered to the Investors and Canadian Investors under the procedures specified
in Paragraphs (a), (b), (c), (d) (e) and (f) of this Section 3.3.

          (h) Each Investor and Canadian Investor may, by prior written consent,
waive its rights under this Section 3.3. Such a waiver shall be deemed a limited
waiver and shall only apply to the extent specifically set forth in the written
consent of such Investor or Canadian Investor.

          3.4. Negative Covenants.

          (a) Supermajority Approvals. The Corporation shall not, directly or
indirectly, take any of the actions specified in Article III, Section A.6(c)(iv)
of the Certificate without (x) the prior written consent or vote of the holders
of at least a majority of the voting power (determined as set forth in the third
sentence of Section A.6(a) of Article III of the Certificate) of the then
outstanding shares of the Series A Preferred Stock, voting or consenting as a
separate class, (y) the prior written consent or vote of the holders of at least
two-thirds of the voting power (determined as set forth in the third sentence of
Section A.6(a) of Article III of the Certificate) of the then outstanding shares
of the B Preferred Stock, voting or consenting together as a separate class, and
(z) the prior written consent or vote of the holders of at least fifty-five
percent (55%) of the voting power (determined as set forth in the third sentence
of Section A.6(a) of Article III of the Certificate) of the then outstanding
shares of the Senior C Preferred Stock, voting or consenting together as a
separate class.

          (b) Majority Approvals. The Corporation shall not, directly or
indirectly, take any of the actions specified in Article III, Section A.6(c)(v)
of the Certificate


                                       13

<Page>

without the prior written consent or vote of the holders of at least a majority
of the voting power (determined as set forth in the third sentence of Section
A.6(a) of Article III of the Certificate) of the then outstanding shares of
Preferred Stock, voting or consenting together as a single class.

          (c) Registration Rights. The Corporation shall not hereafter grant to
any persons any rights to register or qualify stock of the Corporation under
federal or state securities laws, without the prior written consent or vote of
the holders of at least a majority of the then outstanding Restricted Securities
(calculated on an as-converted to Common Stock basis).

          (d) Board Approvals. In addition to and not in limitation of the
provisions of Sections 3.4(a) and (b) hereof, the Corporation shall not,
directly or indirectly, (i) take any of the actions specified in Article V,
Section A.2 of the Certificate without the prior written consent or vote of the
Board of Directors and (ii) take any action (or refrain from taking any action)
as a holder of capital in ABI Canada without the prior written consent or vote
of the Board of Directors (including the affirmative consent or vote of the HCV
Director).

          3.5. Filing of Reports Under the Exchange Act.

          (a) The Corporation shall give prompt notice to the holders of
Preferred Stock and the Canadian Investors of (i) the filing of any registration
statement (an "Exchange Act Registration Statement") pursuant to the Exchange
Act, relating to any class of equity securities of the Corporation, (ii) the
effectiveness of such Exchange Act Registration Statement, and (iii) the number
of shares of such class of equity securities outstanding, as reported in such
Exchange Act Registration Statement, in order to enable the Investors and
Canadian Investors to comply with any reporting requirements under the Exchange
Act or the Securities Act. Upon the written request of (i) Series A Stockholders
holding a majority of the then outstanding shares of Series A Preferred Stock,
voting or consenting together as a separate class, (ii) B Stockholders
representing at least a majority of the voting power (determined as set forth in
the third sentence of Section A.6(a) of Article III of the Certificate) of the
then outstanding shares of the B Preferred Stock, voting or consenting together
as a separate class, or (iii) Senior C Stockholders representing at least
fifty-five percent (55%) of the voting power (determined as set forth in the
third sentence of Section A.6(a) of Article III of the Certificate) of the then
outstanding shares of the Senior C Preferred Stock, voting or consenting
together as a separate class, the Corporation shall, at any time after the
Corporation has registered any shares of Common Stock under the Exchange Act,
file an Exchange Act Registration Statement relating to any class of equity
securities of the Corporation then held by the applicable Investors, or issuable
upon conversion or exercise of any class of debt or equity securities or
warrants or options of the Corporation then held by the applicable Investors,
whether or not the class of equity securities with respect to which such request
is made shall be held by the number of persons which would require the filing of
a registration statement under Section 12(g)(1) of the Exchange Act.

          (b) If the Corporation shall have filed an Exchange Act Registration
Statement or a registration statement (including an offering circular under
Regulation A promulgated under the Securities Act) pursuant to the requirements
of the Securities Act, which


                                       14

<Page>

shall have become effective (and in any event, at all times following the
initial public offering of any of the securities of the Corporation), then the
Corporation shall comply with all of the reporting requirements of the Exchange
Act (whether or not it shall be required to do so) and shall comply with all
other public information reporting requirements of the Commission as a condition
to the availability of an exemption from the Securities Act for the sale of any
of the Restricted Shares by any holder of Restricted Securities (including any
such exemption pursuant to Rule 144 or Rule 144A thereof, as amended from time
to time, or any successor rule thereto or otherwise). The Corporation shall
cooperate with each holder of Restricted Securities in supplying such
information as may be necessary for such holder of Restricted Securities to
complete and file any information reporting forms presently or hereafter
required by the Commission as a condition to the availability of an exemption
from the Securities Act (under Rule 144 or Rule 144A thereunder or otherwise)
for the sale of any of the Restricted Shares by any holder of Restricted
Securities.

          3.6. Access to Records. The Corporation shall afford to each of the
Investors and Canadian Investors and such Investor's and Canadian Investor's
employees, counsel and other authorized representatives, free and full access,
at all reasonable times and for reasonable periods of time, to all of the books,
records and properties of the Corporation and to all officers and employees of
the Corporation.

          3.7. Financial Reports. Until such time that the Corporation has a
class of its equity securities registered under the Exchange Act and is required
to file reports thereunder pursuant to Sections 13 or 15(d) of the Exchange Act,
except with respect to the obligation set forth in Section 3.7(e)(i) hereof
which shall survive such time, the Corporation shall furnish each of the
Investors and Canadian Investors with the financial information described below:

          (a) Within 20 days after the last day of each calendar month (or such
other calendar period as is approved by the Board), financial statements,
including a balance sheet as of the last date of such month, a statement of
income (or monthly operating expenses) for such month, together with a
cumulative statement of income from the first day of the fiscal year to the last
day of such month, which statements shall be prepared from the books and records
of the Corporation, a cash flow analysis, together with cumulative cash flow
analyses from the first day of the fiscal year to the last day of such month,
and a comparison between the actual monthly operating expenses and the projected
figures for such month and the comparable figures for the prior year, subject to
the provisions of Section 3.9 hereof.

          (b) The Corporation shall deliver to each of the Investors and
Canadian Investors, within 45 days after the end of each fiscal quarter,
unaudited financial statements for such fiscal quarter, certified by the Chief
Financial Officer or the Treasurer of the Corporation as presenting fairly the
financial condition and results of operations of the Corporation and as having
been prepared on a basis consistent with the accounting principles reflected in
the Corporation's annual audited financial statements (subject to the absence of
footnotes), accompanied by a report, signed by the Chief Financial Officer or
the Treasurer of the Corporation, summarizing the operating and financial
highlights of the Corporation for such quarter, which report shall include (a) a
comparison between the actual quarterly operating and financial results, the
Budget (as defined in Section 3.8 hereof) and the results of that quarter for


                                       15

<Page>

the prior fiscal year of the Corporation, together with an explanation of
material variances from the Budget and such prior year quarter, as the case may
be, and (b) a narrative analysis of operations and trends in the business of the
Corporation during such quarter.

          (c) Within 90 days after the end of each fiscal year of the
Corporation, audited financial statements of the Corporation, which shall
include an income statement and a statement of cash flow for such fiscal year
and a balance sheet as of the last day thereof, each prepared in accordance with
generally accepted accounting principles consistently applied, and accompanied
by the report of an independent accounting firm of national reputation as shall
have been approved by the Board.

          (d) If for any period the Corporation shall have any subsidiary or
subsidiaries whose accounts are consolidated with those of the Corporation, then
the financial statements delivered for such period pursuant to paragraphs (a),
(b) and (c) of this Section 3.7 shall be the consolidated and consolidating
financial statements of the Corporation for all such consolidated subsidiaries.

          (e) Promptly upon becoming available:

               (i) copies of all financial statements, reports, press releases,
notices, proxy statements and other documents sent by the Corporation to its
Stockholders or released to the public and copies of all regular and periodic
reports, if any, filed by the Corporation with the Commission or any securities
exchange or self-regulatory organization; and

               (ii) any other financial or other information available to
management of the Corporation that any of the Investors and Canadian Investors
shall have reasonably requested on a timely basis.

          3.8. Budget and Operating Forecast. The Corporation shall prepare and
submit to the Board and each of the Investors and Canadian Investors an
operating plan with monthly and quarterly breakdowns (the "Budget") for each
fiscal year at least 45 days prior to the beginning of each fiscal year of the
Corporation. The Budget shall be deemed accepted as the Budget for such fiscal
year only when it has been approved by the Board. The Budget shall be reviewed
by the Corporation periodically and all changes therein, and all material
deviations therefrom, shall be reviewed by the Board on at least a quarterly
basis.

          3.9. System of Accounting. The Corporation shall maintain, and cause
each of its subsidiaries, when and if any shall exist, to maintain, its books of
accounts, related records and system of accounting in accordance with good
business practices and generally accepted accounting principles, and shall cause
the matters contained therein to be appropriately and accurately reflected in
the financial reports (which shall be prepared in accordance with generally
accepted accounting principles) furnished pursuant to this Agreement.

          3.10. Restriction on Transfer Rights; Confidentiality; Termination.
The rights granted to each of the Investors pursuant to Sections 3.6 through 3.8
hereof shall not be transferred or assigned by any Investor to, and shall not
inure to the benefit of, any successor,


                                       16

<Page>

transferee or assignee of any Investor, while it is engaged in any business
directly competitive with the Corporation. The rights granted to each of the
Canadian Investors pursuant to Sections 3.6 through 3.8 hereof shall
automatically terminate upon the date on which such Canadian Investor no longer
owns all of the capital stock in ABI Canada that such Canadian Investor shall
own immediately following the execution of this Agreement.

          3.11. Confidentiality and Non-Competition Agreements for Key
Employees. The Corporation shall cause each person who is presently an employee
of or a consultant or independent contractor to the Corporation or who becomes
an employee of or a consultant to the Corporation subsequent to the date hereof
and who shall have or be proposed to have access to confidential or proprietary
information of the Corporation to execute a confidentiality and non-competition
agreement in form and substance attached hereto or otherwise approved by the
Board prior to the commencement of such person's employment by the Corporation
in such capacity.

          3.12. Stock Restriction Agreement for Chief Officers and Principal
Stockholders. In connection with the transactions contemplated by the Merger
Agreement, the Corporation caused James L. Warren, Glenn Kazo and Steven C.
Gilman, Ph.D. to execute a Third Amended and Restated Stock Restriction
Agreement substantially in the form attached hereto as Exhibit A and shall
maintain such agreement in full force and effect.

          3.13. Marketing and Promotional Material. Each of the Investors and
Canadian Investors will have the right to review and approve, in advance of
publication, distribution or dissemination, any reference to such Investor or
any entity affiliated with such Investor (other than the Corporation), contained
in any document, instrument, report or filing or in any advertising, marketing,
promotional and similar materials.

          3.14. Bring-Along Rights.

          (a) If, by vote or written consent, (i) the Board of Directors, (ii)
the holders of at least a majority of the voting power (determined as set forth
in the third sentence of Section A.6(a) of Article III of the Certificate) of
the then outstanding shares of the Series A Preferred Stock, voting or
consenting as a separate class, (iii) the holders of at least two-thirds of the
voting power (determined as set forth in the third sentence of Section A.6(a) of
Article III of the Certificate) of the then outstanding shares of the B
Preferred Stock, voting or consenting together as a separate class, and (iv) the
holders of at least fifty-five percent (55%) of the voting power (determined as
set forth in the third sentence of Section A.6(a) of Article III of the
Certificate) of the then outstanding shares of the Senior C Preferred Stock,
voting or consenting together as a separate class (the holders referred to in
the foregoing clauses (ii), (iii) and (iv) of this Section 3.14(a) being
referred to herein, collectively, as the "Approving Investors"), approve a
change of control of the Corporation pursuant to which any bona fide
unaffiliated third party proposes to acquire all or substantially all of the
assets or all or substantially all of the capital stock of the Corporation,
whether by purchase, merger, consolidation, share exchange, sale of assets,
exclusive license or otherwise (an "Approved Sale"), the Approving Investors
shall provide all other Investors who are not Approving Investors and each
Original Stockholder (collectively, the "Remaining Stockholders") at least ten
(10) days advance notice of such


                                       17

<Page>

Approved Sale, which notice shall include a reasonably detailed description of
the Approved Sale, including the proposed time and place of closing, the
consideration to be received by the Remaining Stockholders, and any other
material terms. For the avoidance of doubt, in the event that any Canadian
Investor is not an Approving Investor, such Canadian Investor shall be deemed a
Remaining Stockholder for the purposes of this Section 3.14. The Remaining
Stockholders shall consent to, vote for and raise no objections to the Approved
Sale, and (i) the Remaining Stockholders shall waive any dissenters rights,
appraisal rights or similar rights, if any, in connection with such merger,
consolidation or asset sale, or (ii) if the Approved Sale is structured as a
sale of the stock of the Corporation, the Remaining Stockholders shall agree to
sell all of their shares of capital stock of the Corporation on the terms and
conditions approved by the Approving Investors, provided such terms do not
provide that the Remaining Stockholders would receive less than the amount that
would be distributed to such Remaining Stockholders in the event the proceeds of
the Approved Sale were distributed in accordance with the Restated Certificate;
provided, however, that in the event that (a) any Canadian Investor is a
Remaining Stockholder and (b) such Canadian Investor has not exercised the Put
Right, then such Canadian Investor shall agree to sell all of its shares of
capital stock of ABI Canada on the terms and conditions approved by the
Approving Investors. The Remaining Stockholders shall take all reasonably
necessary and desirable actions requested by the Approving Investors in
connection with the consummation of the Approved Sale, including the execution
of such agreements and such instruments (collectively, the "Sale Documents") and
other actions reasonably necessary to (i) effectuate the Approved Sale,
including (only in the case that a third party requires both the Corporation and
all of the Approving Investors and the Remaining Stockholders to individually
sign such Sale Documents) making such customary representations, warranties,
indemnities, covenants, conditions, escrow agreements and other customary
agreements relating to such Approved Sale (provided that each Remaining
Stockholder's aggregate liability pursuant to the Sale Documents or otherwise in
connection with the Approved Sale shall be limited to the value of the
consideration received by each such Remaining Stockholder on account of the
Approved Sale) and (ii) effectuate the agreed-upon allocation and distribution
of the aggregate consideration upon the Approved Sale.

          (b) Each of the Remaining Stockholders hereby appoints, for so long as
the provisions of Section 3.14(a) hereof remain in effect, the Approving
Investor that holds shares of Preferred Stock that, collectively, represent the
largest amount of voting power (determined as set forth in the third sentence of
Section A.6(a) of Article III of the Certificate) as compared to the voting
power (determined as set forth in the third sentence of Section A.6(a) of
Article III of the Certificate) of the shares of Preferred Stock then held by
each other Approving Investors, as such Remaining Stockholder's attorney and
proxy with full power of substitution, to vote, and otherwise act (by written
consent or otherwise) with respect to the capital stock of the Corporation owned
by such Remaining Stockholder, solely on the matters and in the manner specified
in Section 3.14(a) hereof.

          (c) THE PROXIES AND POWER OF ATTORNEY GRANTED PURSUANT TO THE ABOVE
PARAGRAPH ARE IRREVOCABLE AND COUPLED WITH AN INTEREST. Each Remaining
Stockholder hereby revokes all other proxies and powers of attorney on the
matters specified in Section 3.14(a) hereof with respect to the shares of
capital stock of the Corporation which such Remaining Stockholder may have
heretofore appointed or


                                       18

<Page>

granted, and no subsequent proxy or power of attorney shall be given or written
consent executed (and if given or executed, shall not be effective) by such
Remaining Stockholder with respect thereto. All authority herein conferred or
agreed to be conferred shall survive the death or incapacity of each Remaining
Stockholder and any obligation of a Remaining Stockholder under this Agreement
shall be binding upon the heirs, personal representatives and successors of such
Remaining Stockholder.

          3.15. Vesting of Stock. Unless otherwise approved by the Board of
Directors by vote or written consent in which at least one of the Series A
Directors, the B Director and at least one of the Senior C Directors concur, and
except for the Assumed Options (as defined in the Merger Agreement), all stock,
stock options and other stock equivalents issued after the date of this
Agreement to employees, directors, consultants and other service providers
pursuant to a stock purchase or option plan or other employee stock incentive
program or otherwise ("Stock Awards") (i) shall be subject to a minimum vesting
schedule of at least four years, pursuant to which 25% of the shares subject to
any such Stock Award will vest no sooner than one year after the date of the
issuance or grant of such Stock Award, and 75% of the shares subject to such
Stock Award will vest no more frequently and no sooner than monthly on a pro
rata basis beginning on the first anniversary of the date of issuance or grant
of such Stock Award, (ii) shall provide for termination of vesting and
forfeiture of any unvested portion of such Stock Award upon cessation of
employment, and (iii) shall provide that as a condition to issuance of any
shares, the recipient of such shares shall sign a written agreement in a form
approved by the Board providing the Corporation with a right of first refusal on
any transfer of such shares with certain exceptions for transfers to immediate
family members and to trusts of which the holder is the trustee.

          3.16. Environmental Matters. All development, construction and
operation of property purchased, leased or otherwise acquired by the Corporation
shall, in all material respects, comply with, and shall be lawful, permitted and
conforming uses in all material respects under, all applicable building, fire,
safety, subdivision, zoning, sewer, environmental, securities, health, insurance
and other laws, ordinances, rules, regulations and plan approval conditions of
any governmental or public body or authority. The Corporation shall promptly
advise the Investors and Canadian Investors in writing of any pending or
threatened claim, demand or action by any governmental authority or third party
relating to any Hazardous Materials affecting the Property of which it has
knowledge. The Corporation shall not discharge, place, release, spill or dispose
of any Hazardous Materials or any other pollutants or effluents upon the
Property or elsewhere (including, but not limited to, underground injection of
such substances), and the Corporation shall not discharge into the air any
emission which would require a permit under the Clean Air Act or its state
counterparts or any other Environmental Laws, except in compliance with the
Environmental Laws. The Stockholders shall have no control over, or authority
with respect to, the waste disposal operations of the Corporation. The
Corporation hereby indemnifies, defends and holds harmless the Investors and
Canadian Investors from and against any and all manner of actions, causes of
action, suits, debts, accounts, controversies, judgments, claims, demands,
losses or liabilities of any nature (including reasonable attorneys' fees)
directly or indirectly arising out of or attributable to (a) any
misrepresentation or breach of the representations and covenants set forth in
Section 5.18 of the Series C Stock Purchase Agreement, or (b) the use,
generation, storage, release, threatened


                                       19

<Page>

release, discharge, disposal or presence of Hazardous Materials on, under or
about the Property by any person during the period that the Corporation was the
legal or equitable owner of the Property or which occurred prior to such time
and was otherwise actually known by, or should have been known by, the
Corporation. The obligation of the Corporation to indemnify the Investors and
Canadian Investors shall specifically cover and include, without limitation, all
fines and penalties imposed by federal, state or local authorities, costs of
removing or neutralizing the Hazardous Materials, injury to the property
adjoining the Property, injury to persons living or working on or about the
Property or adjoining or otherwise affecting property, and all other indirect or
consequential damages incurred by the Investors and Canadian Investors.

          3.17. Duration of Section. Sections 3.3, 3.4 and 3.6 through 3.16
hereof and the rights and obligations of the parties thereunder shall
automatically terminate (i) on the consummation of a firm commitment
underwritten public offering of Common Stock registered under the Securities Act
pursuant to which (a) Common Stock is offered to the public at a price of at
least $5.00 per share of Common Stock (subject to adjustment for stock splits,
stock dividends, reverse stock splits, stock combinations, recapitalizations and
like occurrences) and (b) the net proceeds to the Corporation are at least $40
million (a "Qualified Public Offering"), and (ii) as to any Investor or Canadian
Investor, when such Investor ceases to own shares of capital stock or other
securities of the Corporation or such Canadian Investor ceases to own shares of
capital stock or other securities of the Corporation or ABI Canada.

     SECTION 4. Transfer of Securities.

          4.1. Restriction on Transfer. The Restricted Securities shall not be
transferable, except upon the conditions specified in this Section 4, which
conditions are intended solely to ensure compliance with the provisions of the
Securities Act in respect of the Transfer thereof.

          4.2. Restrictive Legend. Each certificate evidencing any Restricted
Securities and each certificate evidencing any such securities issued to
subsequent transferees of any Restricted Securities shall (unless otherwise
permitted by the provisions of Section 4.3 or 4.10 hereof) be stamped or
otherwise imprinted with a legend in substantially the following form:

          THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE BEEN ACQUIRED FOR
          INVESTMENT AND HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF
          1933, AS AMENDED, OR ANY STATE SECURITIES LAW. THE SECURITIES MAY NOT
          BE PLEDGED, HYPOTHECATED, SOLD OR TRANSFERRED IN THE ABSENCE OF AN
          EFFECTIVE REGISTRATION STATEMENT FOR THE SECURITIES UNDER THE
          SECURITIES ACT OF 1933 OR ANY APPLICABLE STATE SECURITIES LAW OR AN
          EXEMPTION THEREFROM UNDER SUCH ACT OR LAW. ADDITIONALLY, THE TRANSFER
          OF THESE SECURITIES IS SUBJECT TO THE CONDITIONS SPECIFIED IN THE
          FOURTH AMENDED AND


                                       20

<Page>

          RESTATED STOCKHOLDERS' AGREEMENT DATED DECEMBER 30, 2005 AMONG
          ACTIVBIOTICS, INC. AND CERTAIN OTHER SIGNATORIES THERETO, AS AMENDED
          AND IN EFFECT FROM TIME TO TIME, AND NO TRANSFER OF SUCH SECURITIES
          SHALL BE VALID OR EFFECTIVE UNTIL SUCH CONDITIONS HAVE BEEN FULFILLED.
          COPIES OF SUCH AGREEMENT MAY BE OBTAINED AT NO COST BY WRITTEN REQUEST
          MADE BY THE HOLDER OF RECORD OF THIS CERTIFICATE TO THE SECRETARY OF
          ACTIVBIOTICS, INC.

          4.3. Notice of Transfer. By acceptance of any Restricted Securities,
the holder thereof agrees to give prior written notice to the Corporation of
such holder's intention to effect any Transfer and to comply in all other
respects with the provisions of this Section 4.3. Each such notice shall
describe the manner and circumstances of the proposed Transfer and shall be
accompanied by: (a) the written opinion of counsel for the holder of such
Restricted Securities, or, at such holder's option, a representation letter of
such holder, addressed to the Corporation (which opinion and counsel, or
representation letter, as the case may be, shall be reasonably acceptable to the
Corporation), as to whether, in the case of a written opinion, in the opinion of
such counsel, such proposed Transfer involves a transaction requiring
registration of such Restricted Securities under the Securities Act and
applicable state securities laws or an exemption thereunder is available, or, in
the case of a representation letter, such letter sets forth a factual basis for
concluding that such proposed transfer involves a transaction requiring
registration of such Restricted Securities under the Securities Act and
applicable State Securities laws or that an exemption thereunder is available,
or (b) if such registration is required and if the provisions of Section 4.4
hereof are applicable, a written request addressed to the Corporation by the
holder of such Restricted Securities, describing in detail the proposed method
of disposition and requesting the Corporation to effect the registration of such
Restricted Securities pursuant to the terms and provisions of Section 4.4
hereof; provided, however, that (y) in the case of a Transfer by a holder to a
member of such holder's Group, no such opinion of counsel or representation
letter of the holder shall be necessary, provided that the transferee agrees in
writing to be subject to Sections 4.1, 4.2, 4.3 and 4.10 hereof to the same
extent as if such transferee were originally a signatory to this Agreement, and
(z) in the case of any holder of Restricted Securities that is a partnership, no
such opinion of counsel or representation letter of the holder shall be
necessary for a Transfer by such holder to a partner of such holder, or a
retired partner of such holder who retires after the date hereof, or the estate
of any such partner or retired partner if, with respect to such Transfer by a
partnership, (i) such Transfer is made in accordance with the partnership
agreement of such partnership, and (ii) the transferee agrees in writing to be
subject to the terms of Sections 4.1, 4.2, 4.3 and 4.10 hereof to the same
extent as if such transferee were originally a signatory to this Agreement. If
in such opinion of counsel or as reasonably concluded from the facts set forth
in the representation letter of the holder (which opinion and counsel, or
representation letter, as the case may be, shall be reasonably acceptable to the
Corporation), the proposed Transfer may be effected without registration under
the Securities Act and any applicable state securities laws or "blue sky" laws,
then the holder of Restricted Securities shall thereupon be entitled to effect
such Transfer in accordance with the


                                       21

<Page>

terms of the notice delivered by it to the Corporation. Each certificate or
other instrument evidencing the securities issued upon such Transfer (and each
certificate or other instrument evidencing any such securities not Transferred)
shall bear the legend set forth in Section 4.2 hereof unless: (a) in such
opinion of such counsel or as can be concluded from the representation letter of
such holder (which opinion and counsel or representation letter shall be
reasonably acceptable to the Corporation) the registration of future Transfers
is not required by the applicable provisions of the Securities Act and state
securities laws, or (b) the Corporation shall have waived the requirement of
such legend; provided, however, that such legend shall not be required on any
certificate or other instrument evidencing the securities issued upon such
Transfer in the event such Transfer shall be made in compliance with the
requirements of Rule 144 (as amended from time to time or any similar or
successor rule) promulgated under the Securities Act. The holder of Restricted
Securities shall not effect any Transfer until such opinion of counsel or
representation letter of such holder has been given to and accepted by the
Corporation (unless waived by the Corporation) or until registration of the
Restricted Securities involved in the above-mentioned request has become
effective under the Securities Act. In the event that an opinion of counsel is
required by the registrar or transfer agent of the Corporation to effect a
transfer of Restricted Securities in the future, the Corporation shall seek and
obtain such opinion from its counsel, and the holder of such Restricted
Securities shall provide such reasonable assistance as is requested by the
Corporation (other than the furnishing of an opinion of counsel) to satisfy the
requirements of the registrar or transfer agent to effectuate such transfer.

          4.4. Required Registration. At any time following the date that is the
earlier of (i) the fifth anniversary of the Merger Closing Date (which term
shall have the same meaning herein as the term "Closing Date" has in the Merger
Agreement) and (ii) six months following the closing of an initial public
offering, if the Corporation shall be requested (a) by holders of 15% or more of
the outstanding Restricted Securities (on an as-converted basis) to effect the
registration under the Securities Act of Restricted Shares, or (b) after the
first registration pursuant to this Section 4.4, by one or more of the holders
of Restricted Securities, to effect the registration under the Securities Act of
Restricted Shares, then the Corporation shall promptly give written notice of
such proposed registration to all holders of Restricted Securities, and
thereupon the Corporation shall promptly use its best efforts to effect the
registration under the Securities Act of the Restricted Shares that the
Corporation has been requested to register for disposition as described in the
request of such holders of Restricted Securities and in any response received
from any of the holders of Restricted Securities within 30 days after the giving
of the written notice by the Corporation; provided, however, that the
Corporation shall not be obligated to effect any registration under the
Securities Act except in accordance with the following provisions and Section
4.6 hereof:

          (a) Subject to Section 4.6 hereof, the Corporation shall not be
obligated to file and cause to become effective more than an aggregate of four
(4) registration statements (a maximum of two (2) of which may be initiated by
Senior C Stockholders holding 15% or more of the then outstanding Restricted
Securities (on an as-converted basis) or by B Stockholders holding 15% or more
of the then outstanding Restricted Securities (on an as-converted basis), and
two (2) of which may be initiated by Series A Stockholders holding 15% or more
of the then outstanding Restricted Securities (on an as-converted basis)) in
which


                                       22

<Page>

Restricted Shares are registered under the Securities Act pursuant to this
Section 4.4, if all of the Restricted Shares offered pursuant to such
registration statements are sold thereunder upon the price and terms offered.

          (b) Notwithstanding the foregoing, the Corporation may include in each
such registration requested pursuant to this Section 4.4 any authorized but
unissued shares of Common Stock (or authorized treasury shares) for sale by the
Corporation or any issued and outstanding shares of Common Stock for sale by
others; provided, however, that, if the number of shares of Common Stock so
included pursuant to this clause (b) exceeds the number of Restricted Shares
requested by the holders of Restricted Securities requesting such registration,
then such registration shall be deemed to be a registration in accordance with
and pursuant to Section 4.5 hereof; and provided further, however, that in the
event such registration is pursuant to this Section 4.4, the inclusion of such
previously authorized but unissued shares by the Corporation or issued and
outstanding shares of Common Stock by others in such registration does not
adversely affect, in the sole opinion of the holders of Restricted Securities
requesting such registration, the ability of the holders of Restricted
Securities requesting such registration to market the entire number of
Restricted Shares requested by them.

          4.5. Piggyback Registration.

          (a) Each time that the Corporation proposes for any reason to register
any of its securities under the Securities Act, other than the Corporation's
initial public offering or pursuant to a registration statement on Form S-4 or
Form S-8 or similar or successor forms (collectively, "Excluded Forms"), the
Corporation shall promptly give written notice of such proposed registration to
all holders of Restricted Securities, which shall offer such holders the right
to request inclusion of any Restricted Shares in the proposed registration.

          (b) Each holder of Restricted Securities shall have 30 days from the
receipt of such notice to deliver to the Corporation a written request
specifying the number of Restricted Shares such holder intends to sell and the
holder's intended method of disposition.

          (c) In the event that the proposed registration by the Corporation is,
in whole or in part, an underwritten public offering of securities of the
Corporation, any request under Section 4.5(b) hereof may specify that the
Restricted Shares be included in the underwriting (i) on the same terms and
conditions as the shares of Common Stock, if any, otherwise being sold through
underwriters under such registration, or (ii) on terms and conditions comparable
to those normally applicable to offerings of common stock in reasonably similar
circumstances in the event that no shares of Common Stock other than Restricted
Shares are being sold through underwriters under such registration.

          (d) Upon receipt of a written request pursuant to Section 4.5(b)
hereof, the Corporation shall promptly use its best efforts to cause all such
Restricted Shares to be registered under the Securities Act, to the extent
required to permit sale or disposition as set forth in the written request.


                                       23

<Page>

          (e) Notwithstanding the foregoing, if the managing underwriter of any
such proposed registration determines and advises in writing that the inclusion
of all Restricted Shares proposed to be included in the underwritten public
offering, together with any other issued and outstanding shares of Common Stock
proposed to be included therein by holders other than the holders of Restricted
Securities (such other shares hereinafter collectively referred to as the "Other
Shares"), would interfere with the successful marketing of the Corporation's
securities, then the total number of such securities proposed to be included in
such underwritten public offering shall be reduced. If such proposed
registration is in connection with an offering other than a Qualified Public
Offering, such reduction shall occur (i) first, by the shares requested to be
included in such registration by the holders of Other Shares, and (ii) second,
if necessary, (a) one-half by the securities proposed to be issued by the
Corporation, and (b) one-half by the Restricted Shares proposed to be included
in such registration by the holders thereof, on a pro rata basis, based upon the
number of Restricted Shares sought to be registered by each such holder;
provided however, that in no event shall the number of Restricted Shares to be
included in such offering be reduced to less than 15% of the total shares to be
included therein. The shares of Common Stock that are excluded from the
underwritten public offering pursuant to the preceding sentence shall be
withheld from the market by the holders thereof for a period, not to exceed 180
days from the closing of such underwritten public offering, that the managing
underwriter reasonably determines as necessary in order to effect such
underwritten public offering.

          4.6. Registrations on Form S-3. At such time as the Corporation shall
have qualified for the use of Form S-3 (or any successor form promulgated under
the Securities Act), each holder of Restricted Securities shall have the right
to request in writing an unlimited number of registrations on Form S-3; provided
that the Corporation shall not be required to file more than two such
Registration Statements on Form S-3 pursuant to this Section 4.6 in any twelve
month period. Each such request by a holder shall: (a) specify the number of
Restricted Shares which the holder intends to sell or dispose of, (b) state the
intended method by which the holder intends to sell or dispose of such
Restricted Shares, and (c) request registration of Restricted Shares (including
those Restricted Shares proposed to be registered by persons other than the
initiating holder) having a proposed aggregate offering price of at least
$1,000,000. Upon receipt of a request pursuant to this Section 4.6, the
Corporation shall use its best efforts to effect such registration or
registrations on Form S-3.

          4.7. Preparation and Filing. If and whenever the Corporation is under
an obligation pursuant to the provisions of this Section 4 to use its best
efforts to effect the registration of any Restricted Shares, the Corporation
shall, as expeditiously as practicable:

          (a) prepare and file with the Commission a registration statement with
respect to such securities and use its best efforts to cause such registration
statement to become and remain effective in accordance with Section 4.7(b)
hereof;

          (b) prepare and file with the Commission such amendments and
supplements to such registration statement and the prospectus used in connection
therewith as may be necessary to keep such registration statement effective
until the earlier of (i) the sale of all Restricted Shares covered thereby and
(ii) nine months, and to comply with the provisions of


                                       24

<Page>

the Securities Act with respect to the sale or other disposition of all
Restricted Shares covered by such registration statement;

          (c) furnish to each holder whose Restricted Shares are being
registered pursuant to this Section 4 such number of copies of any summary
prospectus or other prospectus, including a preliminary prospectus, in
conformity with the requirements of the Securities Act, and such other documents
as such holder may reasonably request in order to facilitate the public sale or
other disposition of such Restricted Shares;

          (d) use its best efforts to register or qualify the Restricted Shares
covered by such registration statement under the securities or blue sky laws of
such jurisdictions as each holder whose Restricted Shares are being registered
pursuant to this Section 4 shall reasonably request and do any and all other
acts or things which may be necessary or advisable to enable such holder to
consummate the public sale or other disposition in such jurisdictions of such
Restricted Shares; provided, however, that the Corporation shall not be required
to consent to general service of process for all purposes in any jurisdiction
where it is not then subject to process, qualify to do business as a foreign
corporation where it would not be otherwise required to qualify or submit to
liability for state or local taxes where it is not otherwise liable for such
taxes;

          (e) at any time when a prospectus covered by such registration
statement and relating thereto is required to be delivered under the Securities
Act within the appropriate period mentioned in Section 4.7(b) hereof, notify
each holder whose Restricted Shares are being registered pursuant to this
Section 4 of the happening of any event as a result of which the prospectus
included in such registration, as then in effect, includes an untrue statement
of a material fact or omits to state a material fact required to be stated
therein or necessary to make the statements therein not misleading in the light
of the circumstances then existing and, at the request of such holder, prepare,
file and furnish to such holder a reasonable number of copies of a supplement to
or an amendment of such prospectus as may be necessary so that, as thereafter
delivered to the purchasers of such shares, such prospectus shall not include an
untrue statement of a material fact or omit to state a material fact required to
be stated therein or necessary to make the statements therein not misleading in
the light of the circumstances then existing;

          (f) if the Corporation has delivered preliminary or final prospectuses
to the holders of Restricted Shares that are being registered pursuant to this
Section 4 and after having done so the prospectus is amended to comply with the
requirements of the Securities Act, the Corporation shall promptly notify such
holders and, if requested, such holders shall immediately cease making offers of
Restricted Shares and return all prospectuses to the Corporation. The
Corporation shall promptly provide such holders with revised prospectuses and,
following receipt of the revised prospectuses, such holders shall be free to
resume making offers of the Restricted Shares; and

          (g) furnish, at the request of any holder whose Restricted Shares are
being registered pursuant to this Section 4, on the date that such Restricted
Shares are delivered to the underwriters for sale in connection with a
registration pursuant to this Section 4, if such securities are being sold
through underwriters, or, on the date that the registration statement with


                                       25

<Page>

respect to such securities becomes effective, if such securities are not being
sold through underwriters, (i) an opinion, dated such date, of the counsel
representing the Corporation for the purposes of such registration, in form and
substance as is customarily given to underwriters in an underwritten public
offering, addressed to the underwriters, if any, and to the holder or holders
making such request, and (ii) a letter dated such date, from the independent
certified public accountants of the Corporation, in form and substance as is
customarily given by independent certified public accountants to underwriters in
an underwritten public offering, addressed to the underwriters, if any, and to
the holder or holders making such request.

     Notwithstanding the foregoing, if, after a registration statement becomes
effective, the Corporation becomes engaged in any activity which, in the good
faith determination of the Board, involves information that would have to be
disclosed in the registration statement but which the Corporation desires to
keep confidential for valid business reasons, then the Corporation may at its
option by notice to such holders of Restricted Shares that have included shares
in such registration statement, require that such holders cease sales of such
shares under such registration statement for a period not in excess of ninety
days from the date of such notice, such right to be exercised by the Corporation
not more than once in any 12-month period. If in connection therewith, the
Corporation considers it appropriate for such registration statement to be
amended, the Corporation shall so amend such registration statement as promptly
as practicable and such holders shall suspend any further sales of their shares
until the Corporation advises them that such registration statement has been
amended. The time periods referred to herein during which such registration
statement must be kept effective shall be extended for an additional number of
days equal to the number of days during which the right to sell shares was
suspended pursuant to this paragraph.

          4.8. Expenses. The Corporation shall pay all expenses incurred by the
Corporation in complying with this Section 4, including, without limitation, all
registration and filing fees (including all expenses incident to filing with the
National Association of Securities Dealers, Inc.), fees and expenses of
complying with the securities and blue sky laws of all such jurisdictions in
which the Restricted Shares are proposed to be offered and sold, printing
expenses and fees and disbursements of counsel (including with respect to each
registration effected pursuant to Sections 4.4, 4.5 and 4.6 hereof, the
reasonable fees and disbursements of one counsel for the holders of Restricted
Securities who may be chosen by a majority in interest of the Restricted Shares
being registered in such offering); provided, however, that all underwriting
discounts and selling commissions applicable to the Restricted Shares covered by
registrations effected pursuant to Section 4.4, 4.5 or 4.6 hereof shall be borne
by the seller or sellers thereof, in proportion to the number of Restricted
Shares sold by each such seller or sellers.

          4.9. Indemnification.

          (a) (i) In the event of any registration of any Restricted Shares
under the Securities Act pursuant to this Section 4 or registration or
qualification of any Restricted Shares pursuant to Section 4.7(d) hereof, the
Corporation shall indemnify and hold harmless the seller of such shares, each
underwriter of such shares, if any, each broker or any other person acting on
behalf of such seller and each other person, if any, who controls any of the


                                       26

<Page>

foregoing persons, within the meaning of the Securities Act, against any losses,
claims, damages or liabilities, joint or several, to which any of the foregoing
persons may become subject under the Securities Act or otherwise, insofar as
such losses, claims, damages or liabilities (or actions in respect thereof)
arise out of or are based upon an untrue statement or alleged untrue statement
of a material fact contained in any registration statement under which such
Restricted Shares were registered under the Securities Act, any preliminary
prospectus or final prospectus contained therein, or any amendment or supplement
thereto, or any document incident to registration or qualification of any
Restricted Shares pursuant to Section 4.7(d) hereof, or arise out of or are
based upon the omission or alleged omission to state therein a material fact
required to be stated therein or necessary to make the statements therein not
misleading or, with respect to any prospectus, necessary to make the statements
therein, in light of the circumstances under which they were made, not
misleading, or any violation by the Corporation of the Securities Act or any
state securities or blue sky laws applicable to the Corporation and relating to
action or inaction required of the Corporation in connection with such
registration or qualification under the Securities Act or such state securities
or blue sky laws. The Corporation shall reimburse on demand such seller,
underwriter, broker or other person acting on behalf of such seller and each
such controlling person for any legal or any other expenses reasonably incurred
by any of them in connection with investigating or defending any such loss,
claim, damage, liability or action; provided, however, that the Corporation
shall not be liable in any such case to the extent that any such loss, claim,
damage or liability arises out of or is based upon (a) an untrue statement or
alleged untrue statement or omission or alleged omission made in said
registration statement, preliminary or final prospectus or amendment or
supplement thereto or any document incident to registration or qualification of
any Restricted Shares pursuant to Section 4.7(d) hereof, in reliance upon and in
conformity with written information furnished to the Corporation by such seller,
underwriter, broker, other person or controlling person specifically for use in
the preparation thereof, or (b) if the Corporation provides an amended
prospectus which corrects any misstatement or omission, any use of a prospectus
which does not contain such correction after such correction is made and the
prospectus is provided to such seller.

               (ii) In the event of any registration of any Restricted Shares
under the Securities Act pursuant to this Section 4 or registration or
qualification of any Restricted Shares pursuant to Section 4.7(d) hereof, the
Investors, severally, but not jointly, shall indemnify and hold harmless the
Corporation and its directors, officers, employees, agents and affiliates and
any other person, if any, who controls any of the foregoing persons, within the
meaning of the Securities Act, against any losses, claims, damages or
liabilities (or actions in respect thereof) which arise out of or are based upon
an untrue statement or alleged untrue statement of a material fact contained in
any registration statement under which such Restricted Shares were registered
under the Securities Act, any preliminary prospectus or final prospectus
contained therein, or any amendment or supplement thereto, or any document
incident to registration or qualification of any Restricted Shares pursuant to
Section 4.7(d) hereof, or arise out of or based upon the omission or alleged
omission to state therein a material fact required to be stated therein or
necessary to make the statements therein not misleading or, with respect to any
prospectus, necessary to make statements therein, in light of the circumstances
under which they were made, not misleading, or any violation of the Securities
Act or any state securities or applicable blue sky laws relating to action or
inaction required of the Corporation in connection


                                       27

<Page>

with such registration or qualification under the Securities Act or such state
securities or blue sky laws arising out of or based upon written information
furnished to the Corporation by any such Investor specifically for use in the
preparation thereof. Notwithstanding anything herein to the contrary, no
Investor shall have any obligation hereunder to pay any amount in excess of the
net proceeds received by any such Investor in connection with the registration
of Restricted Shares.

          (b) Before Restricted Shares held by any prospective seller shall be
included in any registration pursuant to this Section 4, such prospective seller
and any underwriter acting on its behalf shall have agreed to indemnify and hold
harmless (in the same manner and to the same extent as set forth in paragraph
(a)) the Corporation, each director of the Corporation, each officer of the
Corporation who signs such registration statement and any person who controls
the Corporation within the meaning of the Securities Act, with respect to any
untrue statement or omission from such registration statement, any preliminary
prospectus or final prospectus contained therein, or any amendment or supplement
thereto, if such untrue statement or omission was made in reliance upon and in
conformity with written information furnished to the Corporation through an
instrument duly executed by such seller or such underwriter specifically for use
in the preparation of such registration statement, preliminary prospectus, final
prospectus or amendment or supplement; provided, however, that the maximum
amount of liability in respect of such indemnification shall be limited, in the
case of each prospective seller, to an amount equal to the net proceeds actually
received by such prospective seller from the sale of Restricted Shares effected
pursuant to such registration.

          (c) Promptly after receipt by an indemnified party of notice of the
commencement of any action involving a claim referred to in Section 4.9(a) or
(b) hereof, such indemnified party will, if a claim in respect thereof is to be
made against the indemnifying party under this Section 4.9, give written notice
to the latter of the commencement of such action. In case any such action is
brought against an indemnified party, the indemnifying party will be entitled to
participate in and to assume the defense thereof, jointly with any other
indemnifying party similarly notified to the extent that it may wish, with
counsel reasonably satisfactory to such indemnified party, and, after notice to
such indemnified party from the indemnifying party of its election to assume the
defense thereof, the indemnifying party shall be responsible for any legal or
other expenses subsequently incurred by the latter in connection with the
defense thereof; provided, however, that, if any indemnified party shall have
reasonably concluded that there may be one or more legal defenses available to
such indemnified party which are different from or additional to those available
to the indemnifying party, or that such claim or litigation involves or could
have an effect upon matters beyond the scope of the indemnity agreement provided
in this Section 4.9, the indemnifying party shall not have the right to assume
the defense of such action on behalf of such indemnified party, and such
indemnifying party shall reimburse such indemnified party and any person
controlling such indemnified party for the fees and expenses of counsel retained
by the indemnified party which are reasonably related to the matters covered by
the indemnity agreement provided in this Section 4.9. The indemnifying party
shall not make any settlement of any claims indemnified against hereunder
without the written consent of the indemnified party or parties, which consent
shall not be unreasonably withheld.


                                       28

<Page>

          (d) In order to provide for just and equitable contribution to joint
liability under the Securities Act in any case in which either (i) any holder of
Restricted Securities exercising rights under this Agreement, or any controlling
person of any such holder, makes a claim for indemnification pursuant to this
Section 4.9, but it is judicially determined (by the entry of a final judgment
or decree by a court of competent jurisdiction and the expiration of time to
appeal or the denial of the last right of appeal) that such indemnification may
not be enforced in such case notwithstanding the fact that this Section 4.9
provides for indemnification in such case, or (ii) contribution under the
Securities Act may be required on the part of any such holder or any such
controlling person in circumstances for which indemnification is provided under
this Section 4.9; then, in each such case, the Corporation and such holder will
contribute to the aggregate losses, claims, damages or liabilities to which they
may be subject as is appropriate to reflect the relative fault of the
Corporation and such holder in connection with the statements or omissions which
resulted in such losses, claims, damages or liabilities, it being understood
that the parties acknowledge that the overriding equitable consideration to be
given effect in connection with this provision is the ability of one party or
the other to correct the statement or omission which resulted in such losses,
claims, damages or liabilities, and that it would not be just and equitable if
contribution pursuant hereto were to be determined by pro rata allocation or by
any other method of allocation which does not take into consideration the
foregoing equitable considerations. Notwithstanding the foregoing, (i) no such
holder will be required to contribute any amount in excess of the proceeds to it
of all Restricted Shares sold by it pursuant to such registration statement, and
(ii) no person or entity guilty of fraudulent misrepresentation, within the
meaning of Section 11(f) of the Securities Act, shall be entitled to
contribution from any person or entity who is not guilty of such fraudulent
misrepresentation.

          (e) Notwithstanding any of the foregoing, if, in connection with an
underwritten public offering of any Restricted Shares, the Corporation, the
holders of such Restricted Shares and the underwriters enter into an
underwriting or purchase agreement relating to such offering which contains
provisions covering indemnification among the parties, then the indemnification
provision of this Section 4.9 shall be deemed inoperative for purposes of such
offering.

          4.10. Removal of Legends, Etc. Notwithstanding the foregoing
provisions of this Section 4, the restrictions imposed by this Section 4 upon
the transferability of any Restricted Securities shall cease and terminate when
(a) any such Restricted Shares are sold or otherwise disposed of in accordance
with the intended method of disposition by the seller or sellers thereof set
forth in a registration statement or such other method contemplated by Section
4.3 hereof that does not require that the securities transferred bear the legend
set forth in Section 4.2 hereof, including a Transfer pursuant to Rule 144 or a
successor rule thereof (as amended from time to time), or (b) the holder of
Restricted Securities has met the requirements for transfer of such Restricted
Securities pursuant to subparagraph (k) of Rule 144 or a successor rule thereof
(as amended from time to time) promulgated by the Commission under the
Securities Act. Whenever the restrictions imposed by this Section 4 have
terminated, a holder of a certificate for Restricted Securities as to which such
restrictions have terminated shall be entitled to receive from the Corporation,
without expense, a new certificate not bearing the restrictive legend set forth
in Section 4.2 hereof and not containing any other reference to the restrictions
imposed by this Section 4.


                                       29

<Page>

     SECTION 5. Securities Act Registration Statements. Except for securities of
the Corporation registered on Excluded Forms, the Corporation shall not file any
registration statement under the Securities Act covering any securities unless
it shall first have given each holder of Restricted Securities written notice
thereof. The Corporation further covenants that each holder of Restricted
Securities shall have the right, at any time when it may be deemed to be a
controlling person of the Corporation, within the meaning of the Securities Act,
to participate in the preparation of such registration statement and to request
the insertion therein of material furnished to the Corporation in writing which
in such holder's judgment should be included. In connection with any
registration statement referred to in this Section 5, the Corporation shall
indemnify, to the extent permitted by law, each holder of Restricted Securities,
its officers, partners and directors and each person, if any, who controls any
such holder within the meaning of the Securities Act in the same manner and to
the same extent as the Corporation is required to indemnify a seller of
Restricted Shares in Section 4.9 hereof. If, in connection with any such
registration statement, any holder of Restricted Securities shall furnish
written information to the Corporation expressly for use in the registration
statement, then such holder shall indemnify the Corporation, each director of
the Corporation, each officer of the Corporation who signs such registration
statement and each person, if any, who controls the Corporation within the
meaning of the Securities Act to the same extent as a seller of Restricted
Shares is required to indemnify such persons in Section 4.9 hereof.

     SECTION 6. Election of Directors.

          6.1. Voting for Directors. At each annual meeting of the stockholders
of the Corporation and at each special meeting of the stockholders of the
Corporation called for the purposes of electing directors of the Corporation,
and at any time at which stockholders of the Corporation shall have the right
to, or shall, vote for or consent to the election of directors, then, in each
such event, each Original Stockholder and each Investor shall vote all shares of
Common Stock, Preferred Stock and any other shares of voting stock of the
Corporation then owned (or controlled as to voting rights) by it, her or him,
whether by purchase, exercise of rights, warrants or options, stock dividends or
otherwise as follows:

          (a) to fix and maintain the number of directors on the Board of
Directors at eleven (11);

          (b) as to Series A Stockholders, pursuant to Paragraph A.6(b)(viii) of
Article III of the Certificate, to elect to the Board two (2) directors
designated by HCV VI (the "Series A Directors"), one of whom must be a
biotechnology industry expert not affiliated with HCV or any member of the HCV
Group (the "Unaffiliated HCV Director"); the Series A Director who is not the
Unaffiliated HCV Director (such Series A Director is sometimes referred to
herein as the "HCV Director") shall also serve on each committee of the Board of
Directors;

          (c) as to B Stockholders, pursuant to Paragraph A.6(b)(vi) of Article
III of the Certificate, to elect to the Board one (1) director designated by the
members of the MDS Group then holding Preferred Stock (the "B Director"), who
shall also serve on the Compensation Committee of the Board of Directors;


                                       30

<Page>

          (d) as to Series C Stockholders, pursuant to Paragraph A.6(b)(i) of
Article III of the Certificate, to elect to the Board one (1) director, who
shall be designated by VenGrowth (the "VenGrowth Director"); and

          (e) as to Series C-2 Stockholders, pursuant to Paragraph A.6(b)(iii)
of Article III of the Certificate, to elect to the Board five (5) directors (the
"Series C-2 Directors"), who shall be designated by holders of a majority of the
shares of Series C-2 Preferred Stock then outstanding, voting as a separate
class (such holders being referred to herein as the "Majority Series C-2
Holders"), provided, however, the right herein of the Majority Series C-2
Holders to designate the Series C-2 Directors shall terminate on [September 29],
2007;

          (f) to elect to the Board one (1) director (the "BioVentures
Director") designated by BioVentures Investors Limited Partnership II
("BioVentures Investors") so long as BioVentures Investors shall own at least
50% of the Series B Preferred Shares that it owns on the date of this Agreement.

          6.2. Post-Merger Agreement Board Slate. Following the consummation of
the transactions contemplated by the Merger Agreement, the Board of Directors
shall be constituted as follows:

          (a) Alan Dunton, as one of the Series C-2 Directors and Chairman of
the Board;

          (b) Steve Gilman, President and Chief Executive Officer;

          (c) John Littlechild, as the HCV Director;

          (d) Mike Hanson, as the Unaffiliated HCV Director

          (e) Walter Gilbert, as the BioVentures Director;

          (f) Gerry Brunk, as the B Director;

          (g) Jeff Courtney, as the VenGrowth Director; and

          (h) Richard U. DeSchutter, as one of the Series C-2 Directors;

          (i) Constantine E. Anagnostopoulos, as one of the Series C-2
Directors;

          (j) Jerry Benjamin, as one of the Series C-2 Directors; and

          (k) Joshua Boger, Ph.D., as one of the Series C-2 Directors.

          6.3. Cooperation of the Corporation. The Corporation shall use its
best efforts to effectuate the purposes of this Section 6, including promoting
the adoption of any necessary amendment of the By-laws of the Corporation and
the Certificate.


                                       31

<Page>

          6.4. Notices. The Corporation shall provide the Investors and the
Canadian Investors with at least twenty (20) days' prior notice in writing of
any intended mailing of notice to the Investors and the Canadian Investors of
the Corporation for a meeting at which directors are to be elected, and such
notice shall include the names of the persons designated by the Corporation
pursuant to this Section 6. HCV VI, the relevant members of the MDS Group,
VenGrowth, the Majority Series C-2 Holders and BioVentures Investors shall each
notify the Corporation in writing at least three (3) days prior to such mailing
of the person(s) respectively designated by it or them pursuant to Section 6.1
hereof as nominees for election to the Board. In the absence of any notice from
HCV VI, the relevant members of the MDS Group, VenGrowth, the Majority Series
C-2 Holders and/or BioVentures Investors, as the case may be, the director(s)
then serving and previously designated by HCV VI, the relevant members of the
MDS Group, VenGrowth, the Majority Series C-2 Holders and/or BioVentures
Investors, as applicable, shall be renominated.

          6.5. Removal. Except as otherwise provided in this Section 6, no
Investor or Original Stockholder shall vote to remove any member of the Board
designated in accordance with the foregoing provisions of this Section 6 unless
the party or parties who designated such director (the "Designating Party")
shall so vote or otherwise consent, and, if the Designating Party shall so vote
or otherwise consent, then the non-designating stockholders shall likewise so
vote. Any vacancy on the Board created by the resignation, removal, incapacity
or death of any person designated under the foregoing provisions of this Section
6 shall be filled by another person designated by the original Designating
Party. Each Series A Stockholder shall vote all voting shares of Series A
Preferred Stock and all other shares of voting stock of the Corporation owned or
controlled by such Series A Stockholder for any such subsequent designee, each B
Stockholder shall vote all voting shares of B Preferred Stock and all other
shares of voting stock of the Corporation owned or controlled by such B
Stockholder for any such subsequent designee, each Senior C Stockholder shall
vote all voting shares of Senior C Preferred Stock and all other shares of
voting stock of the Corporation owned or controlled by such Senior C Stockholder
for any such subsequent designee.

          6.6. Duration of Section. This Section 6 and the rights and
obligations of the parties hereunder shall automatically terminate on the
consummation of a Qualified Public Offering. Prior to such termination the
rights and obligations of any Investor under this Section 6 shall terminate upon
the date on which such Investor no longer owns any Preferred Stock, whereupon
the obligations of the remaining Investors to vote in favor of the designee of
such Investor shall also terminate. The rights and obligations of VenGrowth
under this Section 6 shall terminate upon the date on which VenGrowth no longer
owns any Preferred Stock and any preferred stock in ABI Canada, whereupon the
obligations of the Investors to vote in favor of the designee of VenGrowth shall
also terminate. The rights of BioVentures Investors under this Section 6 shall
also be subject to early termination in accordance with the provisions of
Section 6.1(e) hereof.

     SECTION 7. Amendment to Certificate. Each Original Stockholder and each
Investor hereby agrees to vote all shares of Common Stock, Preferred Stock and
any other shares of voting stock of the Corporation then owned (or controlled as
to voting rights) by it or him, whether by purchase, exercise of rights,
warrants or options, stock dividends or otherwise, and


                                       32

<Page>

each Canadian Investor hereby agrees to exercise its consent and approval rights
under Section 2 hereof, in favor of any amendment to the Certificate to increase
the number of shares of Common Stock authorized for issuance thereunder to the
extent required, necessary or advisable to satisfy the conversion rights under
the Certificate of any series of Preferred Stock, including, without limitation,
the conversion rights (or any adjustment thereto) under the Certificate of the
Series C-2 Preferred Stock as a result of any adjustment to the Series C-2
Conversion Price (as defined in the Certificate) pursuant to Section
A.7(d)(i)(d) of Article III of the Certificate..

     SECTION 8. Indemnification and Insurance.

          8.1. Indemnification of Investors. In the event that any Investor or
Canadian Investor or any director, officer, employee, affiliate or agent thereof
(the "Indemnitees") become involved in any capacity in any action, proceeding,
investigation or inquiry other than a claim by the Corporation against such
Indemnitee in connection with or arising out of any matter related to the
Corporation or any Indemnitee's role or position with the Corporation, the
Corporation shall reimburse each Indemnitee for its legal and other expenses
(including the cost of any investigation and preparation) as they are incurred
by such Indemnitee in connection therewith. The Corporation also agrees to
indemnify each Indemnitee, pay on demand and protect, defend, save and hold
harmless from and against any and all liabilities, damages, losses, settlements,
claims, actions, suits, penalties, fines, costs or expenses (including, without
limitation, attorneys' fees) (any of the foregoing, a "Claim") incurred by or
asserted against any Indemnitee of whatever kind or nature, arising from, in
connection with or occurring as a result of this Agreement or the matters
contemplated by this Agreement. The foregoing agreement shall be in addition to
any rights that any Indemnitee may have at common law or otherwise.
Notwithstanding the foregoing, the Corporation shall not be required to
indemnify any Indemnitee who is found to have acted in a manner demonstrating
willful misconduct or in connection with any claim that such Indemnitee breached
this Agreement unless it is determined that the Indemnitee did not breach this
Agreement.

          8.2. Advancement of Expenses. The Corporation shall advance all
expenses reasonably incurred by or on behalf of the Indemnitees in connection
with any Claim or potential Claim within twenty (20) days after the receipt by
the Corporation of a statement or statements from the Indemnitee requesting such
advance payment or payments from time to time.

          8.3. Life Insurance. The Corporation shall procure and maintain a key
man life insurance policy on the life of Steven C. Gilman, Ph.D. in the face
amount of $1,000,000.00, with the Corporation as the named insured on such
policy, effective as of the date hereof and continuing until such time as such
person no longer serves as a director, officer, employee or consultant of the
Corporation.

     SECTION 9. Remedies. In case any one or more of the covenants and/or
agreements set forth in this Agreement shall have been breached by any party
hereto, the party or parties entitled to the benefit of such covenants or
agreements may proceed to protect and enforce its or their rights, either by
suit in equity and/or action at law, including, but not limited to, an action
for damages as a result of any such breach and/or an action for specific
performance of any such covenant or agreement contained in this Agreement.
Notwithstanding the generality of the


                                       33

<Page>

foregoing, in the event that the Corporation breaches any of its covenants
and/or agreements set forth herein, the Investors shall have the additional
remedy, in their sole discretion, provided that such breach has not been cured
by the later to occur of 15 days after receipt of notice of such breach by the
Corporation or 30 days after the occurrence of such breach, of electing to
immediately exercise their right of redemption set forth in Article III, Section
A.5 of the Certificate, as provided therein, irrespective of whether such right
of redemption otherwise is mature. The rights, powers and remedies of the
parties under this Agreement are cumulative and not exclusive of any other
right, power or remedy which such parties may have under any other agreement or
law. No single or partial assertion or exercise of any right, power or remedy of
a party hereunder shall preclude any other or further assertion or exercise
thereof.

     SECTION 10. Successors and Assigns. Except as otherwise expressly provided
herein, this Agreement shall bind and inure to the benefit of the Corporation
and each of the Original Stockholders, Investors and Canadian Investors and the
respective successors and assigns of the Corporation and each of the Original
Stockholders, Investors and Canadian Investors. None of VenGrowth, BioVentures
Investors, HCV VI or the members of the MDS Group may assign their respective
rights under Section 6.1 hereof to designate nominees for election as directors
of the Corporation without the prior written consent of the Corporation and of a
majority of the then outstanding shares of the Preferred Stock, voting together
as a single class, on an as-converted basis. Subject to the provisions and
requirements of this Section 10 and Section 4 hereof, this Agreement and the
rights and duties of the Investors and Canadian Investors set forth herein may
be freely assigned, in whole or in part (i) by each Investor and Canadian
Investor to any member of such Investor's or Canadian Investor's Group and (ii)
to any other person or entity acquiring Restricted Securities, including any
Restricted Securities that are convertible into Common Stock, equal to no less
than 100,000 shares of Common Stock (subject to adjustment for stock splits,
stock dividends, reverse stock splits, stock combinations, recapitalizations and
like occurrences). Any transferee (other than an Investor or Canadian Investor)
to whom rights under this Agreement are transferred shall, as a condition to the
effectiveness of such transfer, deliver to the Corporation a written instrument
by which such transferee identifies itself, gives the Corporation notice of the
transfer of such rights, identifies the securities of the Corporation owned or
acquired by it and agrees to be bound by the obligations imposed hereunder to
the same extent as if such transferee were an Investor or a Canadian Investor,
as applicable, hereunder. A transferee to whom rights are transferred pursuant
to this Section 10 will be thereafter deemed to be an Investor or Canadian
Investor, as applicable, for the purpose of the execution of such transferred
rights and may not again transfer such rights to any other person or entity,
other than as provided in this Section 10. Neither this Agreement nor any of the
rights or duties of the Corporation or any of the Original Stockholders set
forth herein shall be assigned by the Corporation or by any Original
Stockholder, in whole or in part, without having first received the written
consent or vote of a majority of the then outstanding shares of the Preferred
Stock, voting together as a single class, on an as-converted basis.

     SECTION 11. Duration of Agreement. Except as otherwise set forth herein,
the rights and obligations of the Corporation and each Investor and Canadian
Investor set forth herein shall survive indefinitely, unless and until, by their
respective terms, they are no longer applicable or except as specifically set
forth herein.


                                       34

<Page>

     SECTION 12. Entire Agreement. This Agreement, together with the other
writings referred to herein or delivered pursuant hereto which form a part
hereof, contains the entire agreement among the parties with respect to the
subject matter hereof and amends, restates and supersedes all prior and
contemporaneous arrangements or understandings with respect thereto.

     SECTION 13. Notices. All notices, requests, consents and other
communications 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 mail, addressed or telecopied, as the case
may be, to such party at the address or telecopier number, as the case may be,
set forth below or such other address or telecopier number, as the case may be,
as may hereafter be designated in writing by the addressee to the addressor
listing all parties:

          (i) If to the Corporation, to:

                               ActivBiotics, Inc.
                               110 Hartwell Avenue
                               Lexington, Massachusetts 02421
                               Attention: Chief Executive Officer
                               Telecopier: (781) 274-9129

               with a copy to: Bingham McCutchen LLP
                               150 Federal Street
                               Boston, Massachusetts 02110
                               Attention: Julio E. Vega, Esq.
                               Telecopier: (617) 951-8736

          (ii) If to the Original Stockholders, as set forth on SCHEDULE 1;

          (iii) If to the Series A Stockholders, as set forth on SCHEDULE 2,

               with a copy to: McCarter & English, LLP
                               Four Gateway Center
                               100 Mulberry Street
                               Newark, New Jersey 07102
                               Attention: Jeffrey A. Baumel, Esq.
                               Telecopier: (973) 624-7070

          (iv) If to the Series B Stockholders or Series B-1 Stockholders, as
set forth on SCHEDULE 3 or SCHEDULE 4, as applicable,

               with a copy to: Palmer & Dodge LLP
                               111 Huntington Avenue
                               Boston, MA 02199-7613
                               Attention: James T. Barrett, Esq.


                                       35

<Page>

                               Telecopier: (617) 227-4420

                               McCarter & English, LLP
                               Four Gateway Center
                               100 Mulberry Street
                               Newark, New Jersey 07102
                               Attention: Jeffrey A. Baumel, Esq.
                               Telecopier: (973) 624-7070

          (v) If to the Series C Stockholders, as set forth on SCHEDULE 5,

               with a copy to: McCarter & English, LLP
                               Four Gateway Center
                               100 Mulberry Street
                               Newark, New Jersey 07102
                               Attention: Jeffrey A. Baumel, Esq.
                               Telecopier: (973) 624-7070

                               Palmer & Dodge LLP
                               111 Huntington Avenue
                               Boston, MA 02199-7613
                               Attention: James T. Barrett, Esq.
                               Telecopier: (617) 227-4420

          (vi) If to the Series C-1 Stockholders, as set forth on SCHEDULE 8,

               with a copy to: McCarthy Tetrault LLP
                               Box 48, Suite 4700, TD Bank Tower
                               Toronto, Ontario M5K 1E6
                               Attention: Vanessa Grant
                               Telecopier: (416) 868-0673

          (vii) If to the Series C-2 Stockholders, as set forth on SCHEDULE 6,

               with a copy to: Mintz Levin Cohn Ferris Glovsky and Popeo, P.C.
                               Chrysler Center
                               666 Third Avenue
                               New York, NY 10017
                               Attention: Ivan Blumenthal, Esq.
                               Telecopier: (212) 983-3115


                                       36

<Page>

          (viii) If to the Canadian Investors, as set forth on SCHEDULE 7,

               with a copy to: McCarthy Tetrault LLP
                               Box 48, Suite 4700, TD Bank Tower
                               Toronto, Ontario M5K 1E6
                               Attention: Vanessa Grant
                               Telecopier: (416) 868-0673

     All such notices, requests, consents and communications shall be deemed to
have been received (a) in the case of personal delivery, on the date of such
delivery, (b) in the case of mailing, on the third business day following the
date of such mailing, (c) in the case of overnight mail, on the first business
day following the date of such mailing, and (d) in the case of facsimile
transmission, when confirmed by facsimile machine report.

     SECTION 14. Changes. Except as otherwise provided herein, the terms and
provisions of this Agreement may not be modified or amended, or any of the
provisions hereof waived, temporarily or permanently, or terminated, except
pursuant to the written consent of the Corporation and the holders of a majority
of the shares of the Preferred Shares, voting together as a single class, on an
as-converted basis, provided, however, that (i) any such modification,
amendment, waiver or termination which materially and adversely affects one or
more holders of any series of Preferred Stock (the "Affected Holders") in a
manner different than other holders of such series of the Preferred Stock shall
require the written consent of each such Affected Holder, and (ii) any such
modification, amendment, waiver or termination shall require the written consent
of any Canadian Investor if, but only if, such modification, amendment, waiver
or termination materially and adversely affects any Canadian Investor in a
manner that is different than the manner in which such modification, amendment,
waiver or termination materially and adversely affects the holders of Series C
Preferred Stock. In addition and notwithstanding anything to the contrary set
forth in this Agreement (including, without limitation, this Section 14), (1)
SCHEDULE 6 attached hereto shall be automatically amended from time to time to
include any and all holders of Series C-2 Preferred Stock that have executed and
delivered Letters of Transmittal pursuant to, and in accordance with, the
provisions of the Merger Agreement, and the Corporation shall update and restate
SCHEDULE 6 attached hereto from time to time in order to reflect each automatic
amendment of SCHEDULE 6 pursuant to this clause (1), (2) SCHEDULE 7 attached
hereto shall be automatically amended from time to time to include those holders
of shares of Series C-1 Preferred Stock who acquired such shares of Series C-1
Preferred Stock pursuant to the Put and Support Agreement, and the Corporation
shall update and restate SCHEDULE 7 attached hereto from time to time in order
to reflect each automatic amendment of SCHEDULE 7 pursuant to this clause (2),
(3) SCHEDULE 8 attached hereto shall be automatically amended from time to time
to delete any Canadian Investor that no longer holds any shares of preferred
stock of ABI Canada, and the Corporation shall update and restate SCHEDULE 8
attached hereto from time to time in order to reflect each automatic amendment
of SCHEDULE 8 pursuant to this clause (3), and (4) SCHEDULES 1, 2, 3, 4, 5, 6, 7
and 8 attached hereto shall be automatically amended to include the successors
and permitted assigns of the then-current Original Stockholders, Preferred
Stockholders and Canadian Investors, as the case may be, and the Corporation
shall update and restate any of SCHEDULES 1, 2, 3, 4, 5, 6, 7 and 8 attached
hereto to reflect each automatic amendment of any of such SCHEDULES pursuant to
this clause (4). .


                                       37

<Page>

     SECTION 15. Counterparts. This Agreement may be executed in any number of
counterparts, including by way of Letters of Transmittal, and each such
counterpart shall be deemed to be an original instrument, but all such
counterparts together shall constitute but one agreement.

     SECTION 16. Headings. The headings of the various sections of this
Agreement have been inserted for convenience of reference only and shall not be
deemed to be a part of this Agreement.

     SECTION 17. Nouns and Pronouns. Whenever the context may require, any
pronouns used herein shall include the corresponding masculine, feminine or
neuter forms, and the singular form of names and pronouns shall include the
plural and vice-versa.

     SECTION 18. Severability. Any provision of this Agreement that is
prohibited or unenforceable in any jurisdiction shall, as to such jurisdiction,
be ineffective to the extent of such prohibition or unenforceability without
invalidating the remaining provisions hereof, and any such prohibition or
unenforceability in any jurisdiction shall not invalidate or render
unenforceable such provision in any other jurisdiction.

     SECTION 19. Governing Law. Except to the extent that any provision of this
Agreement is contrary to any mandatory provision of the Delaware General
Corporation Law (in which case such mandatory statutory provision shall apply),
this Agreement shall be governed by and construed in accordance with the laws of
the Commonwealth of Massachusetts and the laws of the United States applicable
therein (without giving effect to any choice or conflict of laws provision or
rule that would cause the application of the laws of any other jurisdiction) and
shall be treated in all respects as a Massachusetts contract.

     SECTION 20. Effectiveness. Notwithstanding any other provision of this
Agreement, this Agreement shall not be effective until the consummation of the
transactions contemplated by the Merger Agreement.

            [The remainder of this page is intentionally left blank.]


                                       38

<Page>

     IN WITNESS WHEREOF the parties hereto have executed this Fourth Amended and
Restated Stockholders' Agreement on the date first above written.

                                    THE CORPORATION:

                                    ACTIVBIOTICS, INC.


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


                                    THE INVESTORS:

                                    MDS LIFE SCIENCES TECHNOLOGY FUND II NC
                                    LIMITED PARTNERSHIP

                                    By: MDS LSTF II (NCGP) Inc.,
                                        its General Partner


                                    By: /s/ R. Lockie  /  /s/ G. Bedell
                                        ------------------------------------
                                    Name:  R. Lockie / G. Bedell
                                    Title: Vice President / Vice President


                                    MDS LIFE SCIENCES TECHNOLOGY FUND II
                                    QUEBEC LIMITED PARTNERSHIP

                                    By: MDS LSTF II (QGP) Inc.,
                                        its General Partner


                                    By: /s/ R. Lockie  /  /s/ G. Bedell
                                        ------------------------------------
                                    Name:  R. Lockie / G. Bedell
                                    Title: Vice President / Vice President


                                    MLII CO-INVESTMENT FUND NC LIMITED
                                    PARTNERSHIP

                                    By: MLII (NCGP) Inc.,
                                        its General Partner


                                    By: /s/ R. Lockie  /  /s/ G. Bedell
                                        ------------------------------------
                                    Name:  R. Lockie / G. Bedell
                                    Title: Vice President / Vice President


                                       39

<Page>

                                    SC BIOTECHNOLOGY DEVELOPMENT FUND LP

                                    By: SC (GP) Inc., its General Partner


                                    By: /s/ Alan Milgate / /s/ William Walmsley
                                          ------------------------------------
                                    Name:  Alan Milgate / William Walmsley
                                    Title: Director / Director


                                    HEALTHCARE VENTURES VI, L.P.

                                    By: HealthCare Partners VI, L.P.
                                        its General Partner


                                    By: /s/ Jeffrey B. Steinberg
                                        ------------------------------------
                                        Jeffrey B. Steinberg
                                        Title: Administrative Partner


                                    DELPHI VENTURES V, L.P.

                                    By: Delphi Management Partners V, LLC
                                    General Partner


                                    By: /s/ David L. Douglas
                                        ------------------------------------
                                    Name: David L. Douglas
                                    Title: Managing Member


                                        DELPHI BIOINVESTMENTS V, L.P.

                                        By: Delphi Management Partners V, LLC
                                        General Partner


                                        By: /s/ David L. Douglas
                                            ------------------------------------
                                        Name: David L. Douglas
                                        Title: Managing Member


                                       40

<Page>

                                        JOHNSON & JOHNSON DEVELOPMENT
                                        CORPORATION


                                        By: /s/ David A. Piacard
                                            ------------------------------------
                                        Name: David A. Piacard
                                        Title: Vice President, Ventures &
                                               Business Development


                                        BIOVENTURES INVESTORS LIMITED
                                        PARTNERSHIP II

                                        By: BioVentures Investors II, LLC,
                                            its General Partner


                                        By: /s/ Marc E. Goldberg
                                            ------------------------------------
                                        Name: Marc E. Goldberg
                                        Title: Managing Director


                                        ROCHE FINANCE LTD

                                        Grenzacherstrasse 124/Bldg 21-292
                                        CH-4070 Basel
                                        Switzerland


                                        By:
                                            ------------------------------------
                                        Name:
                                              ----------------------------------
                                        Title:
                                               ---------------------------------


                                        THE ORIGINAL STOCKHOLDERS:

                                        /s/ Steven C. Gilman, Ph.D.
                                        ----------------------------------------
                                        Steven C. Gilman, Ph.D.

                                        /s/ Glenn Kazo
                                        ----------------------------------------
                                        Glenn Kazo

                                        /s/ James L. Warren
                                        ----------------------------------------
                                        James L. Warren

                                        /s/ Cathy Carew
                                        ----------------------------------------
                                        Cathy Carew


                                       41

<Page>

                                        THE CANADIAN INVESTORS:

                                        CANADIAN MEDICAL DISCOVERIES FUND INC.


                                        By: /s/ Richard Lockie
                                            ------------------------------------
                                        Name: Richard Lockie
                                        Title: Vice President


                                        By: /s/ G. Bedell
                                            ------------------------------------
                                        Name: G. Bedell
                                        Title: Secretary


                                        THE VENGROWTH ADVANCED LIFE SCIENCES
                                        FUND INC.


                                        By: /s/ Jeff Courtney
                                            ------------------------------------
                                        Name: Jeff Courtney
                                        Title: General Partner


                                        By: /s/ Philip Kurtz
                                            ------------------------------------
                                        Name: Philip Kurtz
                                        Title: Senior Legal Counsel


                                       42
<Page>

                                AMENDMENT NO. 1
                           dated as of June 16, 2006
                                     to the
               FOURTH AMENDED AND RESTATED STOCKHOLDERS AGREEMENT

     This AMENDMENT NO. 1 ("AMENDMENT NO. 1") to that certain Fourth Amended and
Restated Stockholders Agreement, dated as of December 30, 2005 (the
"AGREEMENT"), is made as of this 16th day of June, 2006, by and among
ACTIVBIOTICS, INC., a Delaware corporation (the "CORPORATION"), the INVESTORS
(as defined in the Agreement), the CANADIAN INVESTORS (as defined in the
Agreement), HORIZON TECHNOLOGY FUNDING COMPANY II LLC, a Delaware limited
liability company ("HORIZON II"), and HORIZON TECHNOLOGY FUNDING COMPANY III
LLC, a Delaware limited liability company ("HORIZON III"). Unless otherwise
defined herein, capitalized terms used herein shall have the respective meanings
set forth in the Agreement.

     WHEREAS, on the date hereof, the Corporation issued a warrant to each of
Horizon II and Horizon III (each a "WARRANT", and collectively the "WARRANTS")
to purchase up to 61,988 and 61,988 shares of Series C-2 Convertible Preferred
Stock of the Corporation respectively (such shares, the "WARRANT SHARES");

     WHEREAS, pursuant to the terms of their respective Warrants, each of
Horizon II and Horizon III will receive the same piggyback registration rights
for their Warrant Shares as were granted by the Corporation to the Investors in
the Agreement for their Restricted Shares; and

     WHEREAS, the parties to the Agreement wish to amend the Agreement to
account for the grant of piggyback registration rights to each of Horizon II and
Horizon III.

     NOW, THEREFORE, in consideration of the foregoing and the mutual promises
made herein, and for other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the parties agree to amend the
Agreement in accordance with Section 14 thereof as follows:

     1.   INVESTORS. The definition of "INVESTORS" in Section 1 of the Agreement
is hereby deleted in its entirety and replaced with the following:

          "INVESTORS shall have the meaning (severally, but not jointly) set
forth in the preamble hereto. Additionally, except for Sections 4.1, 4.2,
4.3(b), 4.4 and 4.6, each Warrant holder shall be an Investor for all purposes
of Section 4 hereto."

     2.   RESTRICTED SECURITIES. The definition of "RESTRICTED SECURITIES" in
Section 1 of the Agreement is hereby deleted in its entirety and replaced with
the following:

          "RESTRICTED SECURITIES shall mean any of the Preferred Shares and the
Common Stock issued or issuable upon the conversion of the Preferred Shares, all
shares of Common Stock issued or issuable in respect thereof by way of stock
splits, stock dividends, stock combinations, recapitalizations or like
occurrences, and any other shares of Common Stock or other securities of the
Corporation which may be issued hereafter to any of the Investors or any
Canadian Investor or any member of their Group (including without limitation any
shares of Series C-1 Preferred Stock issuable pursuant to the exercise of the
Put Right under the Put and Support Agreement) which are convertible into or
exercisable for shares of Common Stock (including, without limitation, other
classes or series of convertible Preferred Stock, warrants, options or other
rights to purchase Common Stock or convertible debentures or other convertible
debt securities) and the Common Stock issued or issuable upon such conversion or
exercise of such other securities, which have not

<Page>

been sold (a) pursuant to an effective registration statement filed pursuant to
the Securities Act, or (b) pursuant to Rule 144 or Rule 144A promulgated by the
Commission under the Securities Act. Additionally, except for Sections 4.1, 4.2,
4.3(b), 4.4 and 4.6, the Warrant Shares shall be deemed Restricted Securities
for all purposes of Section 4 hereto."

     3.   NO OTHER AMENDMENTS. Except to the extent amended hereby, all of the
definitions, terms, provisions and conditions set forth in the Agreement are
hereby ratified and confirmed and shall remain in full force and effect. The
Agreement and this Amendment No. 1 shall be read and construed together as a
single agreement and the term "Agreement" shall henceforth be deemed a reference
to the Agreement as amended by this Amendment No. 1. This Amendment No. 1 may be
signed in any number of counterparts, each of which shall be deemed to be an
original and all of which together shall constitute one and the same instrument.

            [THE REMAINDER OF THIS PAGE IS INTENTIONALLY LEFT BLANK.]



<Page>

IN WITNESS WHEREOF, the parties have caused this Amendment No. 1 to be
executed and delivered by their duly authorized representatives, all as of
the day and year written above.

<Table>
<Caption>
<S>                                                <C>

                                                   ACTIVBIOTICS, INC.


                                                   By:   /s/ Steve Gilman
                                                        -----------------------------------
                                                   Name:  Steve Gilman
                                                   Title: Chief Executive Officer


                                                   HORIZON TECHNOLOGY FUNDING COMPANY II LLC
                                                   By: Horizon Technology Finance, LLC, its member and agent

                                                   By:   /s/ Robert D. Pomeroy, Jr.
                                                         -----------------------------------
                                                         Robert D. Pomeroy, Jr.
                                                         Managing Member

                                                   HORIZON TECHNOLOGY FUNDING COMPANY III LLC
                                                   By: Horizon Technology Finance, LLC, its member and agent

                                                   By:   /s/ Robert D. Pomeroy, Jr.
                                                         -----------------------------------
                                                         Robert D. Pomeroy, Jr.
                                                         Managing Member

</Table>


<Page>

                                           INVESTORS:
                                           ----------

<Table>
<Caption>
<S>                                                <C>

                                                   MDS LIFE SCIENCES TECHNOLOGY FUND II
                                                   NC LIMITED PARTNERSHIP
                                                   By:  MDS LSTF II (NCGP) Inc., its General Partner

                                                   By: /s/ Stephen Cummings /  /s/ Richard Lockie
                                                       -----------------------------------------------
                                                       Name: Stephen Cummings / Richard Lockie
                                                       Title: Chief Financial Officer / Vice President

                                                   MDS LIFE SCIENCES TECHNOLOGY FUND II
                                                   QUEBEC LIMITED PARTNERSHIP
                                                   By:  MDS LSTF II (QGP) Inc., its General Partner

                                                   By: /s/ Stephen Cummings /  /s/ Richard Lockie
                                                       -----------------------------------------------
                                                       Name: Stephen Cummings / Richard Lockie
                                                       Title: Chief Financial Officer / Vice President

                                                   MLII CO-INVESTMENT FUND NC LIMITED
                                                   PARTNERSHIP
                                                   By:  MLII (NCGP) Inc., its General Partner

                                                   By: /s/ Stephen Cummings /  /s/ Richard Lockie
                                                       -----------------------------------------------
                                                       Name: Stephen Cummings / Richard Lockie
                                                       Title: Chief Financial Officer / Vice President

                                                   SC BIOTECHNOLOGY DEVELOPMENT FUND LP
                                                   By:  SC (GP) Inc., its General Partner

                                                   By: ------------------------------------------------
                                                       Name:
                                                       Title:
</Table>

<Page>

<Table>
<Caption>
<S>                                                <C>

                                                   HEALTHCARE VENTURES VI, L.P.
                                                   By:  HealthCare Partners VI, L.P., its General Partner


                                                   By: /s/ Jeffrey B. Steinberg
                                                       ------------------------------------------------
                                                       Jeffrey B. Steinberg
                                                       Title: Administrative Partner

                                                   HEALTHCARE VENTURES VII, L.P.
                                                   By:  HealthCare Partners VII, L.P., its General Partner


                                                   By: /s/ Jeffrey B. Steinberg
                                                       ------------------------------------------------
                                                       Jeffrey B. Steinberg
                                                       Title: Administrative Partner


                                                   DELPHI VENTURES V, L.P.
                                                   By:  Delphi Management Partners V, LLC, its General Partner

                                                   By: /s/ Deepika R. Pakianathan
                                                       ------------------------------------------------
                                                       Name: Deepika R. Pakianathan
                                                       Title: Attorney-In-Fact


                                                   DELPHI BIOINVESTMENTS V, L.P.
                                                   By:  Delphi Management Partners V, LLC, its General Partner

                                                   By: Deepika R. Pakianathan
                                                       ------------------------------------------------
                                                       Name: Deepika R. Pakianathan
                                                       Title: Attorney-In-Fact


                                                   JOHNSON & JOHNSON DEVELOPMENT CORPORATION

                                                   By:
                                                       ------------------------------------------------
                                                       Name:
                                                       Title:
</Table>


<Page>

<Table>
<Caption>
<S>                                                <C>

                                                   BIOVENTURES INVESTORS LIMITED PARTNERSHIP II
                                                   By:  BioVentures Investors II, LLC, its General Partner

                                                   By:
                                                       -----------------------------------------------------
                                                       Name:  Marc E. Goldberg
                                                       Title:  Managing Director


                                                   ROCHE FINANCE LTD
                                                   Grenzacherstrasse 124/Bldg 21-292
                                                   CH-4070 Basel
                                                   Switzerland

                                                   By:
                                                       -----------------------------------------------------
                                                       Name:
                                                       Title:


                                                   ADVENT MANAGEMENT III LIMITED PARTNERSHIP

                                                   By: /s/ Jerry Benjamin
                                                       -----------------------------------------------------
                                                       Name: Jerry Benjamin
                                                       Title: General Partner


                                                   ADVENT PRIVATE EQUITY FUND III "A" LIMITED PARTNERSHIP

                                                   By: /s/ Jerry Benjamin
                                                       -----------------------------------------------------
                                                       Name: Jerry Benjamin
                                                       Title: General Partner


                                                   ADVENT PRIVATE EQUITY FUND III "B" LIMITED PARTNERSHIP

                                                   By: /s/ Jerry Benjamin
                                                       -----------------------------------------------------
                                                       Name: Jerry Benjamin
                                                       Title: General Partner
</Table>

<Page>
<Table>
<Caption>
<S>                                                <C>
                                                   ADVENT PRIVATE EQUITY FUND III "C" LIMITED PARTNERSHIP

                                                   By: /s/ Jerry Benjamin
                                                       -----------------------------------------------------
                                                       Name: Jerry Benjamin
                                                       Title: General Partner


                                                   ADVENT PRIVATE EQUITY FUND III "D" LIMITED PARTNERSHIP

                                                   By: /s/ Jerry Benjamin
                                                       -----------------------------------------------------
                                                       Name: Jerry Benjamin
                                                       Title: General Partner


                                                   ADVENT PRIVATE EQUITY FUND III AFFILIATES

                                                   By: /s/ Jerry Benjamin
                                                       -----------------------------------------------------
                                                       Name: Jerry Benjamin
                                                       Title: General Partner


                                                   THE ORIGINAL STOCKHOLDERS:

                                                   /s/ Steven C. Gilman, Ph.D.
                                                   ---------------------------------------------------------
                                                   Steven C. Gilman, Ph.D.


                                                   /s/ Glenn Kazo
                                                   ---------------------------------------------------------
                                                   Glenn Kazo


                                                   /s/ James L. Warren
                                                   ---------------------------------------------------------
                                                   James L. Warren

                                                   /s/ Cathy Carew
                                                   ---------------------------------------------------------
                                                   Cathy Carew
</Table>

<Page>

<Table>
<Caption>
<S>                                                <C>
                                                   THE CANADIAN INVESTORS:
                                                   CANADIAN MEDICAL DISCOVERIES FUND INC.


                                                   By:  /s/ Steven Hawkins
                                                        -------------------------------------------------
                                                        Name: Steven Hawkins
                                                        Title: President & CEO

                                                   By:
                                                        -------------------------------------------------
                                                        Name:
                                                        Title:


                                                   THE VENGROWTH ADVANCED LIFE
                                                   SCIENCES FUND INC.


                                                   By:  /s/ Jeff Courtney
                                                        -------------------------------------------------
                                                        Name: Jeff Courtney
                                                        Title: General Partner


                                                   By:  /s/
                                                        -------------------------------------------------
                                                        Name:
                                                        Title:

</Table>



<Page>

                                 AMENDMENT NO.2
                                       TO
               FOURTH AMENDED AND RESTATED STOCKHOLDERS' AGREEMENT

          This AMENDMENT NO. 2 FOURTH AMENDED AND RESTATED STOCKHOLDERS'
AGREEMENT is made as of August 10, 2006 (this "AMENDING AGREEMENT") by and among
ACTIVBIOTICS, INC., a Delaware corporation (the "CORPORATION"), those holders of
Preferred Stock of the Corporation set out on the signature pages attached
hereto.

                                   WITNESSETH:

          WHEREAS, the Corporation and holders of its stock are parties to that
certain Fourth Amended and Restated Stockholders' Agreement (the "STOCKHOLDERS
AGREEMENT"), dated December 30, 2005;

          WHEREAS, section 13 of the Stockholders Agreement provides, among
other things, that it may be amended by the written consent of the Corporation
and the holders of a majority of the shares of the Preferred Shares, voting
together as a single class, on an as-converted basis (the "REQUISITE
MAJORITY"); and

          WHEREAS, the parties hereto constitute the Requisite Majority and both
Requisite Majority and the Corporation have agreed to amend the Stockholders
Agreement, as set forth in this Amending Agreement.

          NOW, THEREFORE, in consideration of the premises and agreements herein
contained, the parties hereto agree as follows:

                                    ARTICLE I

                                   DEFINITIONS

          1.1  TERMS DEFINED IN THIS AGREEMENT. The following terms (whether or
not underscored) when used in this Agreement shall have the following meanings
(such meanings to be equally applicable to the singular and plural forms
thereof):

          "AMENDING AGREEMENT" is defined in the preamble.

          "CORPORATION" is defined in the preamble.

          "REQUISITE MAJORITY" is defined in the recitals.

          "STOCKHOLDERS AGREEMENT" is defined in the recitals.

<Page>

          1.2  OTHER DEFINITIONS. Terms for which meanings are provided in the
Stockholders Agreement are, unless otherwise defined herein or the context
otherwise requires, used in this Amending Agreement with such meanings.

                                    ARTICLE 2

                    AMENDMENTS TO THE STOCKHOLDERS AGREEMENT

          Effective on the date hereof, the Stockholders Agreement is hereby
amended as follows:

          2.1  SECTION 4.4(ii)(a) is hereby deleted in its entirety and the
following is inserted in lieu thereof:

          "(a) by holders of 10% of more of the outstanding Restricted
          Securities (on an as- converted basis) to effect the registration
          under the Securities Act of Restricted Shares, or"

          2.1  Section 4.4(a) is hereby amended by deleting the percentage "15%"
and replacing it with the percentage "10%".

                                    ARTICLE 3

                                  MISCELLANEOUS

          3.1  NO OTHER AMENDMENTS. Except as specifically amended in this
Amending Agreement, the Stockholders Agreement and all other related documents
shall remain in full force and effect and are hereby ratified and confirmed in
all respects.

          3.2  HEADINGS. The headings of the various sections of this Amending
Agreement have been inserted for convenience of reference only and shall not be
deemed to be a part of this Amending Agreement.

          3.3  SUCCESSORS AND ASSIGNS. This Amending Agreement shall be binding
upon and inure to the benefit of the parties hereto and their respect successors
and assigns, except that neither the Borrower nor any Funded Subsidiary may
assign any of its rights or delegate any of its duties hereunder.

          3.4  COUNTERPARTS. This Agreement may be executed in any number of
counterparts, and each counterpart shall be deemed to be an original instrument,
but all such counterparts together shall constitute but one agreement.

          3.5  GOVERNING LAW. Except to the extent that any provision of this
Agreement is contrary to any mandatory provision of the Delaware General
Corporation Law (in which case such mandatory statutory provision shall
apply), this Amending Agreement shall be governed by and construed in
accordance with the laws of the Commonwealth of Massachusetts and the laws of
the United States applicable therein (without giving effect to any choice or
conflict of laws provision or rule that would cause the application of the
laws of any other jurisdiction) and shall be treated in all respects as a
Massachusetts contract.


            [THE REMAINDER OF THIS PAGE IS INTENTIONALLY LEFT BLANK.]



<Page>

        IN WITNESS WHEREOF, the parties hereto have executed this Amending
Agreement as of the date first above written.

<Table>
<Caption>
<S>                                                     <C>
                                                        THE CORPORATION:

                                                        ACTIVBIOTICS, INC.

                                               By:      /s/ Steve Gilman
                                                        ---------------------------------------------------------
                                                        Name: Steve Gilman
                                                        Title: Chief Executive Officer


                                                        THE INVESTORS:

                                                        HEALTHCARE VENTURES VI, L.P.

                                               By:      HealthCare Partners VI, L.P.
                                                        its General Partner

                                               By:      /s/ Jeffrey Steinberg
                                                        ---------------------------------------------------------
                                                        Name: Jeffrey Steinberg
                                                        Title: Administrative Partner


                                                        HEALTHCARE VENTURES VII, L.P.

                                               By:      HealthCare Partners VII, L.P.
                                                        its General Partner

                                               By:      /s/ Jeffrey Steinberg
                                                        ---------------------------------------------------------
                                                        Name: Jeffrey Steinberg
                                                        Title: Administrative Partner


                                                        CANADIAN MEDICAL DISCOVERIES FUND INC.

                                                        By its manager, Medical Discovery Management Corporation

                                               By:      /s/ Steven Hawkins
                                                        ---------------------------------------------------------
                                                        Name: Steven Hawkins
                                                        Title: CEO
</Table>

<Page>
<Table>
<Caption>
<S>                                           <C>
                                                        THE VENGROWTH ADVANCED LIFE SCIENCES FUND INC.

                                               By:      /s/ Jeff Courtney                /s/ Ryan Farquhar
                                                        ---------------------------------------------------------
                                                        Name: Jeff Courtney                  Ryan Farquhar
                                                        Title: General Partner               Legal Counsel


                                                        MDS LIFE SCIENCES TECHNOLOGY FUND II NC LIMITED
                                                        PARTNERSHIP

                                                        By MDS LSIF II (NCGP) Inc., its General Partner

                                               By:      /s/ Stephen Cummings / /s/ Richard Lockie
                                                        ---------------------------------------------------------
                                                        Name: Stephen Cummings / Richard Lockie
                                                        Title: Chief Financial Officer / Vice President


                                                        MDS LIFE SCIENCES TECHNOLOGY FUND II QUEBEC LIMITED

                                                        By MDS LSIF II (QGP) Inc., its General Partner

                                               By:      /s/ Stephen Cummings / /s/ Richard Lockie
                                                        ---------------------------------------------------------
                                                        Name: Stephen Cummings / Richard Lockie
                                                        Title: Chief Financial Officer / Vice President


                                                        MLII CO-INVESTMENT FUND NC LIMITED PARTNERSHIP

                                                        By MLII (NCGP) Inc., its General Partner

                                               By:      /s/ Stephen Cummings / /s/ Richard Lockie
                                                        ---------------------------------------------------------
                                                        Name: Stephen Cummings / Richard Lockie
                                                        Title: Chief Financial Officer / Vice President
</Table>


<Page>

                                 AMENDMENT NO. 3
                          dated as of October 19, 2006
                                     to the
               FOURTH AMENDED AND RESTATED STOCKHOLDERS AGREEMENT

     This AMENDMENT NO. 3 ("AMENDMENT NO. 3") to that certain Fourth Amended and
Restated Stockholders Agreement, dated as of December 30, 2005, as previously
amended by that certain Amendment No. 1 to the Fourth Amended and Restated
Stockholders Agreement, dated as of June 16, 2006, and by that certain Amendment
No. 2 to the Fourth Amended and Restated Stockholders Agreement, dated as of
August 10, 2006 (collectively, the "AGREEMENT"), is made as of this 19th day of
October, 2006, by and among ActivBiotics, Inc., a Delaware corporation (the
"CORPORATION"), and the Investors (as defined in the Agreement). Unless
otherwise defined herein, capitalized terms used herein shall have the
respective meanings set forth in the Agreement.

     WHEREAS, the undersigned persons represent at least a majority of the
shares of the Preferred Shares, voting together as a single class, on an
as-converted basis, which constitutes the persons necessary to effect the
amendments set forth herein; and

     WHEREAS, the Corporation and the Investors wish to amend the Agreement to
modify certain of its terms.

     NOW, THEREFORE, in consideration of the foregoing and the mutual promises
made herein, and for other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the parties agree to amend the
Agreement in accordance with Section 14 thereof as follows:

     1. DURATION OF SECTION 3. Section 3.17 of the Agreement is hereby deleted
in its entirety and replaced with the following:

     "DURATION OF SECTION. Sections 3.3, 3.4 and 3.6 through 3.16 hereof and the
     rights and obligations of the parties thereunder shall automatically
     terminate (i) upon a "Qualified Public Offering", as that term is defined
     in the Corporation's Sixth Amended and Restated Certificate of
     Incorporation, as filed with the Secretary of State of the State of
     Delaware on December 30, 2005, as amended by that Certificate of Amendment
     to the Sixth Amended and Restated Certificate of Incorporation, as filed
     with the Secretary of State of the State of Delaware on March 3, 2006, and
     as may be further amended or restated and in effect from time to time, and
     (ii) as to any Investor or Canadian Investor, when such Investor ceases to
     own shares of capital stock or other securities of the Corporation or such
     Canadian Investor ceases to own shares of capital stock or other securities
     of the Corporation or ABI Canada."

     2. AMENDMENT TO SCHEDULE 4. Schedule 4 to the Agreement is hereby amended
to list SVB Financial Group, a Delaware corporation f/k/a Silicon Valley
Bancshares ("SVB"), but solely for the purpose of acknowledging their receipt of
the rights set forth in Sections 3.5, 4 (excluding those rights set forth in
Sections 4.1, 4.2, 4.3(b), 4.4 and 4.6), 5 and 14 of the Agreement with respect
to any shares which SVB may receive upon exercise of its warrant to purchase
shares of Series B-1 Preferred Stock.

     3. AMENDMENT TO SCHEDULE 6. Schedule 6 to the Agreement is hereby amended
to list each of Horizon Technology Finance II LLC, a Delaware limited liability
company, and Horizon Technology Finance III LLC, , a Delaware limited liability
company, but solely for the purpose of acknowledging their receipt of the rights
set forth in Sections 4 in the Agreement (excluding those rights set forth in
Sections 4.1, 4.2, 4.3(b),



<Page>

4.4 and 4.6) with respect to any shares which they may receive upon exercise of
their respective warrants to purchase shares of Series C-2 Preferred Stock.

     4. NO OTHER AMENDMENTS. Except to the extent amended hereby, all of the
definitions, terms, provisions and conditions set forth in the Agreement are
hereby ratified and confirmed and shall remain in full force and effect. The
Agreement and this Amendment No. 2 shall be read and construed together as a
single agreement, and the term "Agreement" shall henceforth be deemed a
reference to the Agreement as amended by this Amendment No. 2. This Amendment
No. 2 may be signed in any number of counterparts, each of which shall be deemed
to be an original and all of which together shall constitute one and the same
instrument.



            [THE REMAINDER OF THIS PAGE IS INTENTIONALLY LEFT BLANK.]




                                       2

<Page>


     IN WITNESS WHEREOF, the parties have caused this Amendment No. 3 to be
executed and delivered by their duly authorized representatives, all as of the
day and year written above.

<Table>
<Caption>
<S>                                              <C>
                                                 ACTIVBIOTICS, INC.

                                                 By: /s/ Steve Gilman
                                                    --------------------------------------------
                                                 Name:   Steve Gilman
                                                 Title:  Chief Executive Officer

                                                 INVESTORS:

                                                 MDS LIFE SCIENCES TECHNOLOGY FUND II
                                                 NC LIMITED PARTNERSHIP

                                                 By: MDS LSTF II (NCGP) Inc., its General Partner


                                                 By: /s/ G. Bedell   /     /s/ S. Cummings
                                                    --------------------------------------------
                                                 Name:   G. Bedell   /         S. Cummings
                                                 Title:

                                                 MDS LIFE SCIENCES TECHNOLOGY FUND II
                                                 QUEBEC LIMITED PARTNERSHIP

                                                 By: MDS LSTF II (QGP) Inc., its General Partner


                                                 By: /s/ G. Bedell   /     /s/ S. Cummings
                                                    --------------------------------------------
                                                 Name:   G. Bedell   /         S. Cummings
                                                 Title:

                                                  MLII CO-INVESTMENT FUND NC LIMITED
                                                  PARTNERSHIP

                                                  By: MLII (NCGP) Inc., its General Partner

                                                  By: /s/ G. Bedell   /     /s/ S. Cummings
                                                     --------------------------------------------
                                                  Name:   G. Bedell   /         S. Cummings
                                                  Title:

</Table>

<Page>

<Table>
<Caption>
<S>                                              <C>
                                                  SC BIOTECHNOLOGY DEVELOPMENT FUND LP

                                                  By: SC (GP) Inc., its General Partner


                                                  By: /s/ illegible
                                                     --------------------------------------------
                                                  Name:
                                                  Title:


                                                  HEALTHCARE VENTURES VI, L.P.

                                                  By: HealthCare Partners VI, L.P.
                                                      its General Partner


                                                  By: /s/  Jeffrey B. Steinberg
                                                     --------------------------------------------
                                                  Name:  Jeffrey B. Steinberg
                                                  Title: Administrative Partner


                                                  HEALTHCARE VENTURES VII, L.P.

                                                  By: HealthCare Partners VII, L.P.
                                                      its General Partner


                                                  By: /s/ Jeffrey B. Steinberg
                                                     --------------------------------------------
                                                  Name:  Jeffrey B. Steinberg
                                                  Title: Administrative Partner


                                                  DELPHI VENTURES V, L.P.

                                                  By: Delphi Management Partners V, LLC
                                                      General Partner


                                                  By: /s/ David Douglas
                                                     --------------------------------------------
                                                  Name:  David Douglas
                                                  Title: Managing Member

</Table>

                                          2
<Page>

<Table>
<Caption>
<S>                                              <C>
                                                  DELPHI BIOINVESTMENTS V, L.P.

                                                  By: Delphi Management Partners V, LLC
                                                      General Partner


                                                  By: /s/ David Douglas
                                                     --------------------------------------------
                                                  Name:  David Douglas
                                                  Title: Managing Member


                                                  BIOVENTURES INVESTORS LIMITED PARTNERSHIP II

                                                  By: BioVentures Investors II, LLC,
                                                      its General Partner


                                                  By: /s/ Marc E. Goldberg
                                                     --------------------------------------------
                                                  Name:  Marc E. Goldberg
                                                  Title: Managing Director


                                                  ADVENT MANAGEMENT III LIMITED PARTNERSHIP
                                                  ADVENT PRIVATE EQUITY FUND III "A" LIMITED PARTNERSHIP
                                                  ADVENT PRIVATE EQUITY FUND III "B" LIMITED PARTNERSHIP
                                                  ADVENT PRIVATE EQUITY FUND III "C" LIMITED PARTNERSHIP
                                                  ADVENT PRIVATE EQUITY FUND III "D" LIMITED PARTNERSHIP
                                                  ADVENT PRIVATE EQUITY FUND III AFFILIATES


                                                  By: /s/ Jerry Benjamin
                                                     --------------------------------------------
                                                  Name: Jerry Benjamin
                                                  Title: General Partner
</Table>

                                             3
<Page>
<Table>
<Caption>
<S>                                              <C>
                                                  CONSENTED TO AND AGREED:


                                                  CANADIAN MEDICAL DISCOVERIES
                                                         FUND, INC.



                                                  By: /s/ Steven Hawkins
                                                     --------------------------------------------
                                                  Name: Steven Hawkins
                                                  Title:


                                                  THE VENGROWTH ADVANCED
                                                         LIFE SCIENCES FUND INC.



                                                  By: /s/ Jeff Courtney
                                                     --------------------------------------------
                                                  Name: Jeff Courtney
                                                  Title:

</Table>


                                              4