<PAGE>
                                                                   Exhibit 10.21


                                    PAR VALUE

                            STOCK PURCHASE AGREEMENT

                                  BY AND AMONG

                               NOVARTIS PHARMA AG

                                       AND

                          IDENIX PHARMACEUTICALS, INC.

<PAGE>

                                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.........................................................     1
   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.....................................     2
   2.4  Governmental Consents............................................     2
   2.5  Offering.........................................................     3
   2.6  Environmental and Safety Laws....................................     3
   2.7  Investment Company Act...........................................     4
   2.8  Registration Statement...........................................     4
   2.9  Standstill Agreement.............................................     4

3. Representations and Warranties of the Investor........................     4

   3.1  Authorization....................................................     4
   3.2  Purchase Entirely for Own Account................................     4
   3.3  Receipt of Information...........................................     5
   3.4  Investment Experience............................................     5
   3.5  Accredited Investor..............................................     5
   3.6  Restricted Securities............................................     5
   3.7  Legends..........................................................     5

4. Conditions of the Investor's Obligations at Closing...................     5

   4.1  Representations and Warranties...................................     5
   4.2  Performance......................................................     5
   4.3  Qualifications...................................................     6
   4.4  Consummation of the IPO..........................................     6
   4.5  Capital Stock....................................................     6
   4.6  Amended and Restated Stockholders' Agreement.....................     6
   4.7  Material Adverse Effect..........................................     6
   4.8  Legal Opinion....................................................     6
   4.9  Absence of Litigation............................................     6

5. Conditions of the Company's Obligations at Closing....................     6

   5.1  Representations and Warranties...................................     6
   5.2  Qualifications...................................................     6
   5.3  Performance......................................................     7
   5.4  Absence of Litigation............................................     7

6. Miscellaneous.........................................................     7

   6.1  Entire Agreement.................................................     7
</TABLE>


                                        i

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<TABLE>
<S>                                                                           <C>
   6.2  Survival of Warranties...........................................     7
   6.3  Successors and Assigns...........................................     7
   6.4  Governing Law....................................................     7
   6.5  Counterparts.....................................................     7
   6.6  Titles and Subtitles.............................................     7
   6.7  Notices..........................................................     7
   6.8  Finder's Fees....................................................     8
   6.9  Expenses.........................................................     9
   6.10 Attorneys' Fees..................................................     9
   6.11 Amendments and Waivers...........................................     9
   6.12 Severability.....................................................     9
   6.13 Indemnification..................................................     9
   6.14 Remedies.........................................................     9

EXHIBIT

Exhibit A Form of Opinion of Wilmer Cutler Pickering Hale and Dorr LLP
</TABLE>


                                       ii

<PAGE>

                                    PAR VALUE
                            STOCK PURCHASE AGREEMENT

     THIS PAR VALUE STOCK PURCHASE AGREEMENT (this "Agreement") is made as of
the 21st day of July, 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 4,600,000 shares of its Common Stock, par value $0.001 per share (the "Common
Stock");

     WHEREAS, the Investor desires to waive its rights under Section 4.6(a)(ii)
of the Stockholders' Agreement providing for the right to purchase from the
Company at a per share purchase price equal to $0.001, shares of the Company's
Common Stock upon the terms set forth therein (the "Current Option Pool Purchase
Right");

     WHEREAS, in consideration of the Investor's waiver of the Current Option
Pool Purchase Right, the Company shall sell and issue to the Investor 1,100,000
shares of its Common Stock in a private placement to occur concurrently with the
closing of the IPO;

     WHEREAS, the waiver of the Current Option Pool Purchase Right shall be
effective immediately upon the consummation of the transactions contemplated
herein;

     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 1,100,000 shares of Common Stock (the "Shares") at a per share purchase
price equal to $0.001 (the "Purchase Price"). The aggregate purchase price of
the Shares shall be $1,100.

          1.2 CLOSING. The purchase and sale of the Shares shall take place at
the offices of Wilmer Cutler Pickering 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 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

<PAGE>

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
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 June 11, 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

<PAGE>

          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, 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


                                        3

<PAGE>

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 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.

          2.9 STANDSTILL AGREEMENT. The transactions contemplated by this
Agreement (i) have been approved by a majority of the Independent Directors (as
defined in the Stockholders' Agreement) and (ii) do not violate the provisions
of Section 3.4 of the Stockholders' Agreement.

     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.


                                        4

<PAGE>

          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.

          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


                                        5

<PAGE>

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.

          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 4,600,000 shares of its Common Stock in the IPO at a price per share
of fourteen dollars ($14.00). Following the consummation of the IPO, 47,780,018
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 substantially the form filed as Exhibit
10.26 to the Registration Statement) shall be in full force and effect as of the
Closing.

          4.7 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.

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

          4.9 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.

          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


                                        6

<PAGE>

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.

          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


                                        7

<PAGE>

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

          WILMER CUTLER PICKERING HALE AND DORR LLP
          60 State Street
          Boston, MA 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


                                        8

<PAGE>

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 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.


                                       9

<PAGE>

     IN WITNESS WHEREOF, the parties hereto have executed the PAR VALUE STOCK
PURCHASE AGREEMENT as of the date first written above:

IDENIX PHARMACEUTICALS, INC.            NOVARTIS PHARMA AG


By:  /s/ Andrea Corcoran                   By: /s/ Robert E. Pelzer
    ---------------------------------          ---------------------------------
Name:  Andrea Corcoran                     Name:  Robert E. Pelzer
      -------------------------------            -------------------------------
Title: Exec. VP, Legal & Administration    Title:  General Counsel
       ------------------------------             ------------------------------


                                           By:  /s/ Joseph E. Mamie
                                               ---------------------------------
                                           Name:  Joseph E. Mamie
                                                 -------------------------------
                                           Title:  Head Operational Treasury
                                                  ------------------------------

<PAGE>

                                    EXHIBIT A
                               FORM OF OPINION OF
                   WILMER CUTLER PICKERING HALE AND DORR, LLP

July ___, 2004

Novartis Pharma AG
Lichtstrasse 35
CH-4002 Basel
Switzerland

          Re: Idenix Pharmaceuticals, Inc. Stock Sales

Ladies and Gentlemen:

     This opinion is being furnished pursuant to (i) Section 4.9 of the
Concurrent Private Placement Stock Purchase Agreement, dated as of July ___,
2004 (the "CPP Agreement"), by and between Idenix Pharmaceuticals, Inc., a
Delaware corporation (the "Company"), and Novartis Pharma AG (the "Purchaser")
and (ii) Section 4.8 of the Par Value Stock Purchase Agreement (the "PV
Agreement", and, together with the CPP Agreement, the "Agreements"), dated as of
July ___, 2004, by and among the Company and the Purchaser. Capitalized terms
used herein and not otherwise defined shall have the respective meanings
ascribed to them in the Agreements.

     We have acted as counsel to the Company in connection with the preparation,
execution and delivery of the Agreements. As such counsel, we have examined and
are familiar with and have relied upon the following documents:

     (a)  the Restated Certificate of Incorporation and Amended and Restated
          By-laws, each as amended to date, of the Company;

     (b)  a Certificate of the Secretary of State of the State of Delaware,
          dated July ___, 2004, attesting to the continued legal existence and
          corporate good standing of the Company in Delaware (the "Delaware
          Certificate");

     (c)  the Agreements;

     (d)  an Officer's Certificate from the Company, dated as of the date hereof
          (the "Officer's Certificate"), attesting to the Company's charter and
          by-laws, certain resolutions adopted by the Board of Directors of the
          Company, the incumbency of certain officers of the Company, and as to
          certain other matters; and

<PAGE>

     (e)  such other records of meetings, documents, instruments and
          certificates (including but not limited to certificates of public
          officials and officers of the Company) as we have considered necessary
          for purposes of this opinion.

     In our examination of the documents described above, we have assumed the
genuineness of all signatures, the legal capacity of all individual signatories,
the completeness of all corporate and stock records provided to us, the
authenticity of all documents submitted to us as originals, the conformity to
original documents of all documents submitted to us as copies, and the
authenticity of the originals of such latter documents.

     In rendering this opinion, we have relied, as to all questions of fact
material to this opinion, upon certificates of public officials and officers of
the Company and upon the representations and warranties made by the Purchaser
and the Company in the Agreements. We have not attempted to verify independently
such facts. In connection with the preparation of this letter, we have not
conducted a search of any electronic databases or the dockets of any court,
administrative or regulatory body, agency or other filing office in any
jurisdiction.

     For purposes of this opinion, we have assumed that the Agreements have been
duly authorized, executed and delivered by all parties thereto other than the
Company, and that all such other parties have all requisite power and authority
to effect the transactions contemplated by the Agreements. We have also assumed
that each Agreement is the valid and binding obligation of each party thereto
other than the Company and is enforceable against such other parties in
accordance with its terms. We do not render any opinion as to the application of
any federal or state law or regulation to the power, authority or competence of
any party to the Agreements other than the Company.

     For purposes of this opinion we have assumed that the Board of Directors of
the Company has complied with its fiduciary duties in connection with the
transactions contemplated by the Agreements.

     Our opinions set forth below are qualified to the extent that they may be
subject to or affected by (i) applicable bankruptcy, insolvency, reorganization,
moratorium, fraudulent conveyance or similar laws relating to or affecting the
rights of creditors generally, (ii) statutory or decisional law concerning
recourse by creditors to security in the absence of notice or hearing, (iii)
duties and standards imposed on creditors and parties to contracts, including,
without limitation, requirements of good faith, reasonableness and fair dealing,
and (iv) general equitable principles. We express no opinion as to the
availability of any equitable or specific remedy upon any breach of any of the
agreements as to which we are opining herein, or any of the agreements,
documents or obligations referred to therein, or to the successful assertion of
any equitable defenses, inasmuch as the availability of such remedies or the
success of any equitable defense may be subject to the discretion of a court. We
are expressing no opinion herein with respect to compliance by the Company with
state securities or "blue

<PAGE>

sky" laws, or United States federal securities laws, or with any state or
federal securities antifraud laws.

     We also express no opinion herein as to any provision of any agreement (a)
which may be deemed to or construed to waive any right of the Company, (b) to
the effect that rights and remedies are not exclusive, that every right or
remedy is cumulative and may be exercised in addition to or with any other right
or remedy and does not preclude recourse to one or more other rights or
remedies, (c) relating to the effect of invalidity or unenforceability of any
provision of the Agreements on the validity or enforceability of any other
provision thereof, (d) requiring the payment of penalties, consequential damages
or liquidated damages, (e) which is in violation of public policy, including,
without limitation, any provision relating to indemnification and contribution
with respect to securities law matters, (f) purporting to indemnify any person
against his, her or its own negligence or intentional misconduct, (g) which
provides that the terms of the Agreements may not be waived or modified except
in writing or (h) relating to choice of law or consent to jurisdiction.

     Our opinion expressed in paragraph 1 below, insofar as it relates to the
due organization, valid existence and good standing of the Company, is based
solely on the Delaware Certificate and is limited accordingly, and, as to such
matters, our opinion is rendered as of the date of such certificate. We express
no opinion as to the tax good standing of the Company in any jurisdiction.

     For purposes of our opinions in paragraphs 4 and 5 below, we have relied
upon representations made by the Purchaser in Section 3 of each of the
Agreements, and have assumed (without any independent investigation) the
accuracy of such representations.

     We are opining herein solely as to the state laws of the Commonwealth of
Massachusetts, the Delaware General Corporation Law statute and the federal laws
of the United States of America. To the extent that any other laws govern any of
the matters as to which we are opining below, we have assumed, with your
permission and without independent investigation, that such laws are identical
to the state laws of the Commonwealth of Massachusetts, and we express no
opinion as to whether such assumption is reasonable or correct. We note that the
Agreements state that they are governed by Delaware law.

     For purposes of our opinions rendered below, we have assumed that the facts
and law governing the future performance by the Company of its obligations under
the Agreements will be identical to the facts and law governing its performance
on the date of this opinion.

     Based upon and subject to the foregoing, we are of the opinion that:

     1.   The Company is a corporation duly organized, validly existing and in
          good standing under the laws of the State of Delaware and has all
          requisite corporate power and authority to conduct its business and to
          own, lease and operate its properties as such business and properties
          are described in

<PAGE>

          the prospectus forming a part of the Registration Statement on Form
          S-1 (File No. 333-111157) filed by the Company with the Securities and
          Exchange Commission under the Securities Act of 1933, as amended, on
          December 15, 2003, as filed with the Commission pursuant to Rule
          424(b) of the Securities Act on July ___, 2004, to enter into and
          perform its obligations under each of the Agreements, and to carry out
          the transactions contemplated by the Agreements.

     2.   The Shares have been duly authorized by all necessary corporate action
          on the part of the Company; and the Shares, when issued, sold and
          delivered against payment therefor in accordance with the provisions
          of each of the Agreements, will be duly and validly issued, fully paid
          and non-assessable.

     3.   The execution and delivery by the Company of each of the Agreements,
          and the consummation by the Company of the transactions contemplated
          thereby, have been duly authorized by all necessary corporate action
          on the part of the Company, and each of the Agreements has been duly
          executed and delivered by the Company. Each of the Agreements
          constitutes the valid and binding obligation of the Company,
          enforceable against the Company in accordance with its terms.

     4.   The execution and delivery by the Company of the Agreements, and the
          consummation by the Company of the transactions contemplated thereby
          and the issuance of the Shares, do not and will not (A) conflict with
          or constitute a breach or violation of any of the terms or provisions
          of, or a default under, the Company's Restated Certificate of
          Incorporation or Amended and Restated By-laws, in each case as in
          effect as of the time of the execution and delivery of the Agreements
          and as in effect on the date hereof, or any indenture, loan agreement,
          mortgage, lease or other agreement or instrument to which the Company
          or any of its subsidiaries is a party and that is filed as an exhibit
          to the Registration Statement on Form S-1 (File No. 333-111157) filed
          by the Company with the Securities and Exchange Commission under the
          Securities Act of 1933, as amended, on December 15, 2003, and
          pre-effective amendments numbered 1, 2, 3, 4 and 5 thereto, or (B)
          except with respect to United States federal or Massachusetts state
          securities laws, violate or conflict with any United States federal or
          Massachusetts state law, rule or regulation that in our experience is
          normally applicable in transactions of the type contemplated by the
          Agreements, the Delaware General Corporation Law statute, or any
          judgment, order or decree specifically naming the Company or Idenix
          (Massachusetts), Inc. of which we are aware.

     5.   No consent, approval, authorization or other order of, or
          qualification with, any United States federal or Massachusetts state
          governmental authority or agency is necessary for the issuance, sale
          and delivery of the Shares by the Company to Novartis pursuant to each
          of the Agreements,

<PAGE>

          except such as have been made or obtained (other than the filing of a
          Form D under the Securities Act of 1933, as amended, and the filings
          under applicable state securities laws).

     This opinion is provided to the Purchaser as a legal opinion only and not
as a guaranty or warranty of the matters discussed herein. This opinion is based
upon currently existing statutes, rules, regulations and judicial decisions and
is rendered as of the date hereof, and we disclaim any obligation to advise you
of any change in any of the foregoing sources of law or subsequent developments
in law or changes in facts or circumstances which might affect any matters or
opinions set forth herein.

     This opinion is rendered only to the Purchaser and is solely for the
benefit of the Purchaser in connection with the transactions contemplated by the
Agreements. This opinion may not be relied upon by the Purchaser for any other
purpose, nor may this opinion be provided to, quoted to or relied upon by any
other person or entity for any purpose without our prior written consent.

Best regards,

WILMER CUTLER PICKERING
HALE AND DORR LLP


By:
    ---------------------------------
                  , a Partner
    --------------