<PAGE>


                                                                   EXHIBIT 10.30




                          CONCURRENT PRIVATE PLACEMENT

                            STOCK PURCHASE AGREEMENT

                                  by and among

                               NOVARTIS PHARMA AG

                                       and

                          IDENIX PHARMACEUTICALS, INC.

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                                TABLE OF CONTENTS

<TABLE>
<CAPTION>
                                                                                                         PAGE
                                                                                                         ----
<S>                                                                                                      <C>
1.     Purchase and Sale of Stock.....................................................................     1

   1.1      Sale and Issuance of the Shares...........................................................     1
   1.2      Closing...................................................................................     1
   1.3      Delivery..................................................................................     2
   1.4      Further Assurances........................................................................     2

2.     Representations and Warranties of the Company..................................................     2

   2.1      Organization; Good Standing; Qualification................................................     2
   2.2      Authorization.............................................................................     2
   2.3      Valid Issuance of the Shares..............................................................     3
   2.4      Governmental Consents.....................................................................     3
   2.5      Offering..................................................................................     3
   2.6      Environmental and Safety Laws.............................................................     3
   2.7      Investment Company Act....................................................................     4
   2.8      Registration Statement....................................................................     4

3.     Representations and Warranties of the Investor.................................................     5

   3.1      Authorization.............................................................................     5
   3.2      Purchase Entirely for Own Account.........................................................     5
   3.3      Receipt of Information....................................................................     5
   3.4      Investment Experience.....................................................................     5
   3.5      Accredited Investor.......................................................................     6
   3.6      Restricted Securities.....................................................................     6
   3.7      Legends...................................................................................     6

4.     Conditions of the Investor's Obligations at Closing............................................     6

   4.1      Representations and Warranties............................................................     6
   4.2      Performance...............................................................................     6
   4.3      Qualifications............................................................................     6
   4.4      Consummation of the IPO...................................................................     7
   4.5      Capital Stock.............................................................................     7
   4.6      Amended and Restated Stockholders' Agreement..............................................     7
   4.7      Enterprise Value..........................................................................     7
   4.8      Material Adverse Effect...................................................................     7
   4.9      Legal Opinion.............................................................................     7
   4.10     Absence of Litigation.....................................................................     7

5.     Conditions of the Company's Obligations at Closing.............................................     7

   5.1      Representations and Warranties............................................................     7
   5.2      Qualifications............................................................................     8
   5.3      Performance...............................................................................     8
   5.4      Absence of Litigation.....................................................................     8
</TABLE>

                                        i
<PAGE>

<TABLE>
<S>                                                                                                       <C>
6.     Miscellaneous..................................................................................     8

   6.1      Entire Agreement..........................................................................     8
   6.2      Survival of Warranties....................................................................     8
   6.3      Successors and Assigns....................................................................     8
   6.4      Governing Law.............................................................................     8
   6.5      Counterparts..............................................................................     8
   6.6      Titles and Subtitles......................................................................     9
   6.7      Notices...................................................................................     9
   6.8      Finder's Fees.............................................................................    11
   6.9      Expenses..................................................................................    11
   6.10     Attorneys' Fees...........................................................................    11
   6.11     Amendments and Waivers....................................................................    11
   6.12     Severability..............................................................................    11
   6.13     Indemnification...........................................................................    11
   6.14     Remedies..................................................................................    12
</TABLE>

EXHIBIT

Exhibit A   Form of Opinion of Hale and Dorr LLP

                                       ii
<PAGE>

                          CONCURRENT PRIVATE PLACEMENT
                            STOCK PURCHASE AGREEMENT

      THIS CONCURRENT PRIVATE PLACEMENT STOCK PURCHASE AGREEMENT (this
"Agreement") is made as of the [ ] day of [ ], 2004, by and among IDENIX
PHARMACEUTICALS, INC., a Delaware corporation (the "Company"), and NOVARTIS
PHARMA AG (the "Investor").

                                    RECITALS

      WHEREAS, the Company and the Investor, among other parties, are parties to
that certain Stockholders' Agreement, dated as of May 8, 2003, (the
"Stockholders' Agreement");

      WHEREAS, the Company is undertaking an initial public offering (the "IPO")
of up to [______] shares of its Common Stock, par value $0.001 per share (the
"Common Stock");

      WHEREAS, pursuant to Section 4.7(d) of the Stockholders' Agreement, the
Investor has the right to acquire a pro rata portion of the shares of Common
Stock to be offered in the IPO such that Novartis and its affiliates (other than
Novartis BioVentures Limited) shall be able to maintain after such IPO the same
proportionate ownership interest in the Company as they held before the IPO;

      WHEREAS, the Company has authorized the sale and issuance of [ ] shares of
its Common Stock to the Investor in a private placement to occur concurrently
with the closing of the IPO; and

      WHEREAS, the Investor desires to purchase such shares on the terms and
conditions set forth herein.

      NOW, THEREFORE, in consideration of the foregoing recitals and the mutual
promises hereinafter set forth, the parties hereto agree as follows:

      1.    PURCHASE AND SALE OF STOCK.

            1.1   SALE AND ISSUANCE OF THE SHARES. Subject to the terms and
conditions of this Agreement, the Investor agrees to purchase at the Closing (as
defined below) and the Company agrees to sell and issue to the Investor at the
Closing up to [ ] shares of Common Stock (the "Shares") at a per share purchase
price equal to $[ ] (the "Purchase Price").

            1.2   CLOSING. The purchase and sale of the Shares shall take place
at the offices of Hale and Dorr LLP, 60 State Street, Boston, Massachusetts
02129, concurrently with the closing of the Company's IPO as contemplated by the
Company's Registration Statement on Form S-1 (File No. 333-111157), as amended,
and the

<PAGE>

Underwriting Agreement, dated the date hereof, by and between the Company and
the Representatives of the Several Underwriters named therein (the "Underwriting
Agreement") (which time and place are designated as the "Closing"). The date of
the Closing is referred to herein as the "Closing Date".

            1.3   DELIVERY. At the Closing, subject to the terms and conditions
hereof, the Company will cause to be delivered to the Investor a facsimile
certificate representing the number of Shares to be purchased at the Closing by
the Investor and the Company will cause its transfer agent to promptly deliver
such certificate to the Investor not later than one business day after the
Closing Date, against receipt from the Investor of the Purchase Price for the
Shares by wire transfer made payable to the order of the Company.

            1.4   FURTHER ASSURANCES. The Company and the Investor hereby
covenant and agree without the necessity of any further consideration, to
execute, acknowledge and deliver any and all such other documents and take any
such other action as may be reasonably necessary to carry out the intent and
purposes of this Agreement.

      2.    REPRESENTATIONS AND WARRANTIES OF THE COMPANY. The Company hereby
represents and warrants to the Investor as follows:

            2.1   ORGANIZATION; GOOD STANDING; QUALIFICATION. The Company is a
corporation duly organized, validly existing, and in good standing under the
laws of the State of Delaware, has all requisite corporate power and authority
to own and operate its properties and assets and to carry on its business as now
conducted and as presently proposed to be conducted, to execute and deliver this
Agreement, to issue and sell the Shares and to carry out the provisions of this
Agreement. The Company is duly qualified and is authorized to transact business
and is in good standing as a foreign corporation in each jurisdiction in which
the nature of its business or property makes such qualification necessary,
except where the failure so to qualify would have a material adverse effect on
the condition (financial or otherwise), results of operations, assets,
liabilities or business of the Company and its subsidiaries, taken as a whole,
other than changes, effects or circumstances (i) that are the result of factors
generally affecting the pharmaceutical industry or (ii) that are attributable to
the announcement or performance of this Agreement or the consummation of the
transactions contemplated by this Agreement (a "Material Adverse Effect").

            2.2   AUTHORIZATION. All corporate action on the part of the
Company, its officers, directors and stockholders necessary for the
authorization, execution and delivery of this Agreement, the performance of all
obligations of the Company hereunder at the Closing and the authorization,
issuance, sale and delivery of the Shares has been taken or will be taken prior
to the Closing, and this Agreement, when executed and delivered by the Company
and each other party thereto will constitute a valid and legally binding
obligation of the Company, enforceable in accordance with its terms except (i)
as limited by applicable bankruptcy, insolvency, reorganization, moratorium, and
other laws of general application affecting enforcement of creditors' rights
generally and (ii) as

                                        2
<PAGE>

limited by public policy and laws relating to the availability of specific
performance, injunctive relief, or other equitable remedies.

            2.3   VALID ISSUANCE OF THE SHARES. The Shares, when issued, sold
and delivered in accordance with the terms of this Agreement for the
consideration expressed herein, will be duly and validly issued, fully paid and
nonassessable, and will be free of restrictions on transfer imposed by or
through the Company other than restrictions on transfer under this Agreement or
the Lock-Up Agreement, dated [ ], 2004, by and between the Investor and the
Representatives of the Several Underwriters named therein.

            2.4   GOVERNMENTAL CONSENTS. No consent, approval, qualification,
order or authorization of, or filing with, any local, state, or federal
governmental authority is required on the part of the Company in connection with
the Company's valid execution, delivery or performance of this Agreement, or the
offer, sale or issuance of the Shares by the Company, except such filings as
have been made prior to the Closing and except any notices of sale required to
be filed with the Securities and Exchange Commission under Regulation D of the
Securities Act of 1933, as amended (the "Securities Act").

            2.5   OFFERING. Based in part on the truth and accuracy of the
Investor's representations set forth in this Agreement, the offer, sale and
issuance of the Shares as contemplated by this Agreement are exempt from the
registration requirements of the Securities Act, and neither the Company nor any
authorized agent acting on its behalf will take any action hereafter that would
cause the loss of such exemption.

            2.6   ENVIRONMENTAL AND SAFETY LAWS.

            (a)   Neither the Company nor any Subsidiary (as defined below) is
in material violation of any Environmental Law (as defined below) and, to its
knowledge, no material expenditures are or will be required in order to comply
with any Environmental Law. As used in this Agreement, "Environmental Law" shall
mean any applicable federal, state and local law, ordinance, rule or regulation
of any nation or government whether federal, state, municipal, local,
provincial, regional or other political subdivision thereof that regulates,
fixes liability for, or otherwise relates to, the environment or workplace
health and safety, including, without limitation, the handling, use (including
use in industrial processes, in construction, as building materials, or
otherwise), treatment, storage presence, actual Release (as defined below) or
threatened Release (whether by disposal, a discharge into any water source or
system or into the air, or otherwise) of any Hazardous Materials (as defined
below). The term "Release" shall mean any spilling, leaking, pumping, pouring,
emitting, emptying, discharging, injecting, escaping, leaching, migrating,
dumping or disposing into the environment. The term "Subsidiary" shall mean any
and all corporations, partnerships, joint ventures, associations and other
entities controlled by the Company directly or indirectly through one or more
intermediaries.

            (b)   Neither the Company nor any Subsidiary has used, generated,
manufactured, refined, treated, transported, stored, handled, transferred,
produced,

                                       3
<PAGE>

processed or Released any Hazardous Materials on, from or affecting any Property
(as defined below) in any manner or by any means in material violation of any
Environmental Laws and, to the Company's knowledge, there is no threat of such
use, generation, manufacture, refining, treatment, transportation, storage,
handling, transfer, production, processing or Release. As used herein, the term
"Property" shall include, without limitation, land, buildings and laboratory
facilities owned or leased or otherwise used by the Company or any Subsidiary or
as to which the Company or such Subsidiary now has any duties, responsibilities
(for cleanup, remedy or otherwise) or liabilities under any Environmental Laws,
or as to which the Company or any Subsidiary may have such duties,
responsibilities or liabilities because of past acts or omissions of the Company
or any such Subsidiary or their predecessors, or because the Company 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 or laboratory
facilities. The term "Hazardous Materials" shall mean any flammable explosives,
petroleum products, petroleum by-products, radioactive materials, hazardous
wastes, hazardous substances, toxic substances, pollutants, contaminants, or
other materials regulated or defined as such by Environmental Laws.

            (c)   The Company has not received written notice that it or any
Subsidiary is a party potentially responsible for environmental cleanup costs
incurred at a site or for corrective action under any Environmental Laws. The
Company has not received any written requests for information in connection with
any inquiry by any Governmental Entity (as defined below) concerning disposal
sites or other environmental matters. The term "Governmental Entity" shall mean
any federal, state, local, municipal or foreign court of competent jurisdiction,
governmental agency, authority, instrumentality, regulatory body, stock market
or stock exchange.

            (d)   The stockholders of the Company have had no control over, or
authority with respect to, and have not exercised control over the waste
disposal operations of the Company or any Subsidiary. The Company has not
exercised control over the waste disposal operations of any Subsidiary.

            2.7   INVESTMENT COMPANY ACT. The Company is not an "investment
company", or a company "controlled" by an "investment company", within the
meaning of the Investment Company Act of 1940, as amended.

            2.8   REGISTRATION STATEMENT. The Registration Statement on Form S-1
(File No. 333-111157) (the "Registration Statement") conforms, and its final
prospectus in the form first filed pursuant to Rule 424(b) under the Securities
Act (the "Prospectus") and any further amendments or supplements to the
Registration Statement or the Prospectus will conform, in all material respects
to the requirements of the Securities Act and the rules and regulations of the
Securities and Exchange Commission thereunder and do not and will not, as of the
applicable effective date as to the Registration Statement and any amendment
thereto, and as of the applicable filing date as to the Prospectus and any
amendment or supplement thereto, contain 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. The Company assumes full
responsibility for the

                                       4
<PAGE>

accuracy and completeness of all information included in the Registration
Statement and acknowledges that the Investor has relied upon the Registration
Statement in entering into this Agreement; provided, however, that this
representation and warranty shall not apply to any statements or omissions made
in reliance upon and in conformity with information furnished in writing to the
Company by the Investor expressly for use in the Registration Statement.

      3.    REPRESENTATIONS AND WARRANTIES OF THE INVESTOR. The Investor hereby
represents and warrants to the Company that:

            3.1   AUTHORIZATION. The Investor has full power and authority to
enter into this Agreement, and that this Agreement, when executed and delivered,
will constitute a valid and legally binding obligation of the Investor. All
corporate action on the part of the Investor, its officers, directors and
stockholders necessary for the authorization, execution and delivery of this
Agreement and the performance of all obligations of the Investor hereunder at
the Closing has been taken or will be taken prior to the Closing, and this
Agreement, when executed and delivered, will constitute a valid and legally
binding obligation of the Investor, enforceable in accordance with its terms
except (i) as limited by applicable bankruptcy, insolvency, reorganization,
moratorium, and other laws of general application affecting enforcement of
creditors' rights generally and (ii) as limited by laws relating to the
availability of specific performance, injunctive relief, or other equitable
remedies.

            3.2   PURCHASE ENTIRELY FOR OWN ACCOUNT. The Shares to be purchased
by the Investor will be acquired for investment for the Investor's own account,
not as a nominee or agent, and not with a view to the resale or distribution of
any part thereof, and that the Investor has no present intention of selling,
granting any participation in, or otherwise distributing the same. By executing
this Agreement, the Investor further represents that the Investor does not have
any contract, undertaking, agreement or arrangement with any person to sell,
transfer or grant participations to such person or to any third person, with
respect to any of the Shares.

            3.3   RECEIPT OF INFORMATION. The Investor believes the Investor has
received all the information the Investor considers necessary or appropriate for
deciding whether to purchase the Shares. The Investor further represents that
the Investor has had an opportunity to ask questions and receive answers from
the Company regarding the terms and conditions of the offering of the Shares and
the business, properties, prospects and financial condition of the Company. The
foregoing, however, does not limit or modify the representations and warranties
of the Company in Section 2 of this Agreement or the right of the Investor to
rely thereon.

            3.4   INVESTMENT EXPERIENCE. The Investor represents that the
Investor is experienced in evaluating and investing in private placement
transactions of securities of companies in a similar stage of development and
acknowledges that the Investor is able to fend for itself, can bear the economic
risk of the Investor's investment, and has such knowledge and experience in
financial and business matters that the Investor is capable of evaluating the
merits and risks of the investment in the Shares.

                                       5
<PAGE>

            3.5   ACCREDITED INVESTOR. The Investor either (i) is an "accredited
investor" (as defined in Regulation D under the Securities Act) or (ii) is a
"non-U.S. person" (as defined in Regulation S under the Securities Act) and is
not acquiring the Shares for the account or benefit of any U.S. Person.

            3.6   RESTRICTED SECURITIES. The Investor understands that the
Shares may not be sold, transferred, or otherwise disposed of without
registration under the Securities Act or an exemption therefrom, and that in the
absence of an effective registration statement covering the Shares or an
available exemption from registration under the Securities Act, the Shares must
be held indefinitely. In particular, the Investor is aware that the Shares may
not be sold pursuant to Rule 144 promulgated under the Securities Act unless all
of the conditions of that Rule are met.

            3.7   LEGENDS. To the extent applicable, each certificate evidencing
any of the Shares shall be endorsed with the legends set forth below:

            "THE SHARES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE
            SECURITIES ACT OF 1933, AS AMENDED, AND MAY NOT BE SOLD,
            TRANSFERRED, ASSIGNED, PLEDGED OR HYPOTHECATED UNLESS AND UNTIL
            REGISTERED UNDER SUCH ACT, OR AN EXEMPTION THEREFROM UNDER SUCH
            ACT."

      4.    CONDITIONS OF THE INVESTOR'S OBLIGATIONS AT CLOSING. The obligations
of the Investor under this Agreement are subject to the fulfillment on or before
the Closing of each of the following conditions, the waiver of which shall not
be effective against the Investor if the Investor does not consent in writing
thereto:

            4.1   REPRESENTATIONS AND WARRANTIES. The representations and
warranties of the Company contained in Section 2 shall be true on and as of the
Closing with the same effect as though such representations and warranties had
been made on and as of the date of the Closing. An officer of the Company shall
have provided the Investor an executed certificate certifying that the
representations and warranties of the Company contained in Section 2 are true
and correct as of the date of the Closing.

            4.2   PERFORMANCE. The Company shall have performed and complied
with all agreements, obligations and conditions contained in this Agreement that
are required to be performed or complied with by it on or before the Closing. An
officer of the Company shall have provided the Investor an executed certificate
certifying that the conditions specified in this Section 4.2 have been
fulfilled.

            4.3   QUALIFICATIONS. All authorizations, approvals, or permits, if
any, of any governmental authority or regulatory body of the United States or of
any state that are required in connection with the lawful issuance and sale of
the Shares pursuant to this Agreement shall be duly obtained and effective as of
the Closing.

                                       6
<PAGE>

            4.4   CONSUMMATION OF THE IPO. The IPO shall have been consummated
and the First Time of Delivery (as defined in the Underwriting Agreement) shall
have occurred.

            4.5   CAPITAL STOCK. Upon the consummation of the IPO, the Company
shall have sold [ ] shares of its Common Stock in the IPO at a price per share
of [ ] dollars ($[ ]). Following the consummation of the IPO, [ ] shares of the
Company's Common Stock shall be outstanding and, after giving effect to the
purchase of the Shares pursuant to this Agreement, the Investor shall own not
less than 54% of the voting stock of the Company. An officer of the Company
shall have provided the Investor an executed certificate certifying that each
condition set forth in this Section 4.5 is true and correct as of the date of
the Closing.

            4.6   AMENDED AND RESTATED STOCKHOLDERS' AGREEMENT. The Amended and
Restated Stockholders' Agreement (in the form of Exhibit 10.26 to the
Registration Statement) shall be in full force and effect as of the Closing.

            4.7   ENTERPRISE VALUE. The pre-money enterprise value of the
Company as determined by the Representatives of the Several Underwriters named
in the Underwriting Agreement equals or exceeds $500 million.

            4.8   MATERIAL ADVERSE EFFECT. There shall not have occurred any
event, occurrence, or circumstance which has had or could be reasonably likely
to have a Material Adverse Effect.

            4.9   LEGAL OPINION. The Investor shall have received from Hale and
Dorr LLP such firm's legal opinion addressed to the Investor and dated the
Closing Date, which shall be substantially in the form attached hereto as
Exhibit A.

            4.10  ABSENCE OF LITIGATION. There shall be no injunction, action,
suit proceeding or investigation pending or currently threatened against the
Company or the Investor which questions the validity of this Agreement or the
right of the Company or the Investor to enter into this Agreement or to
consummate the transaction contemplated hereby.

      5.    CONDITIONS OF THE COMPANY'S OBLIGATIONS AT CLOSING. The obligations
of the Company under this Agreement are subject to the fulfillment on or before
the Closing of each of the following conditions, the waiver of which shall not
be effective against the Company if the Company does not consent in writing
thereto:

            5.1   REPRESENTATIONS AND WARRANTIES. The representations and
warranties of the Investor contained in Section 3 shall be true on and as of the
Closing with the same effect as though such representations and warranties had
been made on and as of the date of the Closing. An officer of the Investor shall
have provided the Company an executed certificate certifying that the
representations and warranties of the Investor contained in Section 3 of this
Agreement are true and correct as of the date of the Closing.

                                       7
<PAGE>

            5.2   QUALIFICATIONS. All authorizations, approvals, or permits, if
any, of any governmental authority or regulatory body of the United States or of
any state that are required in connection with the lawful issuance and sale of
the Shares pursuant to this Agreement shall be duly obtained and effective as of
the Closing.

            5.3   PERFORMANCE. The Investor shall have performed and complied
with in all material respects all agreements, obligations and conditions
contained in this Agreement that are required to be performed or complied with
by it on or before the Closing. An officer of the Investor shall have provided
the Company an executed certificate certifying that the conditions specified in
this Section 5.3 have been fulfilled.

            5.4   ABSENCE OF LITIGATION. There shall be no injunction, action,
suit proceeding or investigation pending or currently threatened against the
Company or the Investor which questions the validity of this Agreement or the
right of the Company or the Investor to enter into this Agreement or to
consummate the transaction contemplated hereby.

      6.    MISCELLANEOUS.

            6.1   ENTIRE AGREEMENT. This Agreement and the documents referred to
herein constitute the entire agreement among the parties relating to the subject
matter hereof and thereof.

            6.2   SURVIVAL OF WARRANTIES. The warranties, representations and
covenants of the Company and the Investor contained in or made pursuant to this
Agreement shall survive the execution and delivery of this Agreement and the
Closing.

            6.3   SUCCESSORS AND ASSIGNS. Except as otherwise provided herein,
the terms and conditions of this Agreement shall inure to the benefit of and be
binding upon the respective successors and assigns of the parties (including
permitted transferees of the Shares). Nothing in this Agreement, express or
implied, is intended to confer upon any party other than the parties hereto or
their respective successors and assigns any rights, remedies, obligations or
liabilities under or by reason of this Agreement, except as expressly provided
in this Agreement. Notwithstanding anything to the contrary contained herein,
the Investor's rights and obligations under this Agreement may be assigned to an
Affiliate (as defined in the Amended and Restated Stockholders' Agreement),
provided that the Company is given at least one (1) business day prior written
notice of the assignment by the Investor.

            6.4   GOVERNING LAW. This Agreement shall be governed by and
construed under the laws of the State of Delaware, without reference to
conflicts of laws principles.

            6.5   COUNTERPARTS. This Agreement may be executed in two or more
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

                                       8
<PAGE>

            6.6   TITLES AND SUBTITLES. The titles and subtitles used in this
Agreement are used for convenience only and are not to be considered in
construing or interpreting this Agreement.

            6.7   NOTICES. Unless otherwise provided, all notices and other
communications required or permitted under this Agreement shall be in writing
and shall be mailed by United States first-class mail, postage prepaid, sent by
facsimile or delivered personally by hand or by a nationally recognized courier
addressed to the party to be notified at the address or facsimile number
indicated for such person on the signature page hereof, or at such other address
or facsimile number as such party may designate by ten (10) days' advance
written notice to the other parties hereto. All such notices and other written
communications shall be effective on the date of mailing, facsimile transfer or
delivery. Unless otherwise provided in writing to the other party, all notices
shall be sent to the following addresses:

      NOVARTIS PHARMA AG
      Lichtstrasse 35
      CH-4002 Basel
      Switzerland
      Attention: Chief Executive Officer
      Telecopy: 41-61-324-6677

      With a copy to (which shall not constitute notice):

            NOVARTIS PHARMA AG
            Lichtstrasse 35
            CH-4002 Basel
            Switzerland
            Attention: General Counsel
            Telecopy: 41-61-324-6859

            DEWEY BALLANTINE LLP
            1301 Avenue of the Americas
            New York, NY 10019
            Attention: Morton A. Pierce, Esq.
            Facsimile: (212) 259-6333

      IDENIX PHARMACEUTICALS, INC.
      60 Hampshire Street
      Cambridge, Massachusetts 02139
      Attention: Chief Executive Officer
      Telecopy: (617) 995-9801

      With a copy to (which shall not constitute notice):

            IDENIX PHARMACEUTICALS, INC.
            60 Hampshire Street

                                       9
<PAGE>

            Cambridge, Massachusetts 02139
            Attention: General Counsel
            Telecopy: (617) 995-9801

                                       10
<PAGE>

            HALE AND DORR LLP
            60 State Street
            Boston, Massachusetts 02129
            Attention: Susan W. Murley, Esq.
            Telecopy: (617) 526-5000

            6.8   FINDER'S FEES. Each party represents that it neither is nor
will be obligated for any finder's fee or commission in connection with this
transaction.

      The Investor agrees to indemnify and to hold harmless the Company from any
liability for any commission or compensation in the nature of a finder's fee
(and the cost and expenses of defending against such liability or asserted
liability) for which the Investor or any of its officers, partners, employees or
representatives is responsible.

      The Company agrees to indemnify and hold harmless the Investor from any
liability for any commission or compensation in the nature of a finder's fee
(and the costs and expenses of defending against such liability or asserted
liability) for which the Company or any of its officers, employees or
representatives is responsible.

            6.9   EXPENSES. Irrespective of whether the Closing is effected,
each party hereto shall pay all costs and expenses incurred by them with respect
to the negotiation, execution, delivery and performance of this Agreement.

            6.10  ATTORNEYS' FEES. If any action at law or in equity is
necessary to enforce or interpret the terms of this Agreement, the prevailing
party shall be entitled to reasonable attorneys' fees, costs and disbursements
in addition to any other relief to which such party may be entitled.

            6.11  AMENDMENTS AND WAIVERS. Any term of this Agreement may be
amended, and the observance of any term of this Agreement may be waived (either
generally or in a particular instance and either retroactively or
prospectively), only with the written consent of the party to be charged. Any
amendment or waiver effected in accordance with this paragraph shall be binding
upon each holder of any securities purchased under this Agreement at the time
outstanding, each future holder of all such securities and the Company.

            6.12  SEVERABILITY. If one or more provisions of this Agreement are
held to be unenforceable under applicable law, such provision shall be excluded
from this Agreement and the balance of the Agreement shall be interpreted as if
such provision were so excluded and shall be enforceable in accordance with its
terms.

            6.13  INDEMNIFICATION. The Company shall indemnify, defend and hold
the Investor and the Investor's directors, officers, employees, agents and
affiliates harmless against any and all liabilities, loss, cost or damage,
together with all reasonable costs and expenses related thereto (including legal
and accounting fees and expenses), arising from, relating to, or connected with
the untruth, inaccuracy or breach of any

                                       11
<PAGE>

statements, representations, warranties or covenants of the Company contained
herein or in any certificate provided by the Company pursuant to this Agreement.
The foregoing indemnification shall survive the termination of this Agreement
for any reason.

            6.14  REMEDIES. In case any one or more of the covenants 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 their rights either by suit in
equity or action at law, including, but not limited to, an action for damages as
a result of any such breach or any action for specific performance of any such
covenant or agreement contained in this Agreement. The rights, powers and
remedies of the parties under this Agreement are cumulative an 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 or any right, power
or remedy or a party hereunder shall preclude any other or further assertion or
exercise thereof.

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<PAGE>

      IN WITNESS WHEREOF, the parties hereto have executed the CONCURRENT
PRIVATE PLACEMENT STOCK PURCHASE AGREEMENT as of the date first written above:

IDENIX PHARMACEUTICALS, INC.                         NOVARTIS PHARMA AG
By:_____________________                             By:________________________

Name:___________________                             Name:______________________

Title:__________________                             Title:_____________________

                                                     By:________________________

                                                     Name:______________________

                                                     Title:_____________________

<PAGE>

                                    EXHIBIT A
                      FORM OF OPINION OF HALE AND DORR, LLP


1.    Idenix Pharmaceuticals, Inc. (the "Company") is a corporation duly
      incorporated, validly existing and in good standing under the laws of the
      State of Delaware and has all requisite corporate power and authority to
      own its properties and conduct its business.

2.    The Shares have been duly and validly authorized and issued and are fully
      paid and nonassessable.

3.    The Concurrent Private Placement Stock Purchase Agreement constitutes the
      valid and binding obligation of the Company, enforceable against the
      Company in accordance with its terms.

4.    The execution, delivery and performance by the Company of the Concurrent
      Private Placement Stock Purchase Agreement, the issue and sale of the
      Shares, the compliance by the Company with all of the provisions of the
      Concurrent Private Placement Stock Purchase Agreement and the consummation
      by the Company of the transactions contemplated thereby, do not and will
      not conflict with or result in a breach or violation of any of the terms
      or provisions of, or constitute a default under, and will not conflict
      with or result in a breach or violation of any of the terms or provisions
      of, or constitute a default under, any indenture, mortgage, deed of trust,
      loan agreement or other agreement or instrument known to such counsel to
      which the Company or any of its subsidiaries is a party or by which the
      Company or any of its subsidiaries is bound or to which any of the
      property or assets of the Company or any of its subsidiaries is subject
      that is required by the Securities Act or the rules and regulations
      thereunder to be filed as an exhibit to the Registration Statement, nor
      will such action result in any violation of the provisions of the Restated
      Certificate of Incorporation or Amended and Restated By-Laws of the
      Company or any applicable U.S. Federal or Massachusetts State law or the
      Delaware General Corporation Law statute or any order, rule, regulation,
      judgment or decree of any court or governmental agency or body having
      jurisdiction over the Company or any of its subsidiaries or any of their
      properties, which in such counsel's experience is applicable in
      transactions of the type contemplated by the Concurrent Private Placement
      Stock Purchase Agreement.

5.    No consent, approval, authorization, order, registration or qualification
      of or with any court or governmental agency or body is required for the
      issue and sale of the Shares or the consummation by the Company of the
      transactions contemplated by the Concurrent Private Placement Stock
      Purchase Agreement under the laws of the Commonwealth of Massachusetts,
      the Delaware General Corporation Law and the federal laws of the United
      States of America, except such consents, approvals, authorizations,
      registrations or qualifications as may be required under state securities
      or Blue Sky laws in connection with the purchase and distribution of the
      Shares.

<PAGE>

6.    The offer, issuance and sale of the Shares pursuant to the Concurrent
      Private Placement Stock Purchase Agreement are exempt from registration
      under the Securities Act.