Document:

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                                                                   EXHIBIT 10.32

                          IDENIX PHARMACEUTICALS, INC.

                            2004 STOCK INCENTIVE PLAN

1.    Purpose

      The purpose of this 2004 Stock Incentive Plan (the "Plan") of Idenix
Pharmaceuticals, a Delaware corporation (the "Company"), is to advance the
interests of the Company's stockholders by enhancing the Company's ability to
attract, retain and motivate persons who make (or are expected to make)
important contributions to the Company by providing such persons with equity
ownership opportunities and performance-based incentives and thereby better
aligning the interests of such persons with those of the Company's stockholders.
Except where the context otherwise requires, the term "Company" shall include
any of the Company's present or future parent or subsidiary corporations as
defined in Sections 424(e) or (f) of the Internal Revenue Code of 1986, as
amended, and any regulations promulgated thereunder (the "Code") and any other
business venture (including, without limitation, joint venture or limited
liability company) in which the Company has a controlling interest, as
determined by the Board of Directors of the Company (the "Board").

2.    Eligibility

      All of the Company's employees, officers, directors, consultants and
advisors are eligible to be granted options, restricted stock awards, or other
stock-based awards (each, an "Award") under the Plan. Each person who has been
granted an Award under the Plan shall be deemed a "Participant."

3.    Administration and Delegation

      (a)   Administration by the Compensation Committee. Unless otherwise
determined by the Board, the Plan will be administered by the Compensation
Committee of the Board, provided, however, both the Compensation Committee and
the Board shall each have authority to grant Awards and to adopt, amend and
repeal such administrative rules, guidelines and practices relating to the Plan
as it shall deem advisable. The Board and the Compensation Committee may correct
any defect, supply any omission or reconcile any inconsistency in the Plan or
any Award in the manner and to the extent it shall deem expedient to carry the
Plan into effect and it shall be the sole and final judge of such expediency.
All decisions by the Board and/or the Compensation Committee shall be made in
their respective sole discretion and shall be final and binding on all persons
having or claiming any interest in the Plan or in any Award. No director or
person acting pursuant to the authority delegated to the Compensation Committee
shall be liable for any action or determination relating to or under the Plan
made in good faith. References in the Plan to the Compensation Committee shall
be deemed references to the Board to the extent such reference is being made in
connection with a grant of an Award by the Board.

      (b)   Appointment of Committees. To the extent permitted by applicable
law, the Compensation Committee may delegate any or all of its powers under the
Plan to one or more committees or subcommittees of the Compensation Committee (a
"Committee"). All references in the Plan to the "Compensation Committee " shall
mean the Compensation Committee or a Committee of the Compensation Committee to
the extent that the Compensation Committee 's powers or authority under the Plan
have been delegated to such Committee.

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4.    Stock Available for Awards

      (a)   Number of Shares. Subject to adjustment under Section 8, Awards may
be made under the Plan for up to 800,000 shares of common stock, $0.001 par
value per share, of the Company (the "Common Stock"). In addition to the
foregoing limitation, no more than 300,000 shares of Common Stock (subject to
adjustment pursuant to Section 9 hereof) may be issued pursuant to all Awards
other than Options and SARs (each as defined herein). If any Award expires or is
terminated, surrendered or canceled without having been fully exercised or is
forfeited in whole or in part (including as the result of shares of Common Stock
subject to such Award being repurchased by the Company at the original issuance
price pursuant to a contractual repurchase right) or results in any Common Stock
not being issued, the unused Common Stock covered by such Award shall again be
available for the grant of Awards under the Plan, subject, however, in the case
of Incentive Stock Options, to any limitations under the Code. Shares issued
under the Plan may consist in whole or in part of authorized but unissued shares
or treasury shares.

      (b)   Per-Participant Limit. Subject to adjustment under Section 8, for
Options and SARs granted after the Common Stock is registered under the
Securities Exchange Act of 1934 (the "Exchange Act"), the maximum number of
shares of Common Stock with respect to which Awards may be granted to any
Participant under the Plan shall be 800,000 per calendar year. The
per-Participant limit described in this Section 4(b) shall be construed and
applied consistently with Section 162(m) of the Code ("Section 162(m)").

5.    Stock Options

      (a)   General. The Compensation Committee may grant options to purchase
Common Stock (each, an "Option") and determine the number of shares of Common
Stock to be covered by each Option, the exercise price of each Option and the
conditions and limitations applicable to the exercise of each Option, including
conditions relating to applicable federal or state securities laws, as it
considers necessary or advisable. An Option which is not intended to be an
Incentive Stock Option (as hereinafter defined) shall be designated a
"Nonstatutory Stock Option."

      (b)   Incentive Stock Options. An Option that the Compensation Committee
intends to be an "incentive stock option" as defined in Section 422 of the Code
(an "Incentive Stock Option") shall only be granted to employees of Idenix
Pharmaceuticals, Inc., and its present or future parent or subsidiary
corporations as defined in Sections 424(e) or (f) of the Code, and any other
entities the employees of which are eligible to receive Incentive Stock Options
under the Code, and shall be subject to and shall be construed consistently with
the requirements of Section 422 of the Code. The Company shall have no liability
to a Participant, or any other party, if an Option (or any part thereof) that is
intended to be an Incentive Stock Option is not an Incentive Stock Option.

      (c)   Exercise Price. The Compensation Committee shall establish the
exercise price at the time each Option is granted and specify it in the
applicable option agreement, provided, however, in no event shall the exercise
price be less than the fair market value of the Company's Common Stock on the
date of grant as determined by (or in a manner approved by) the Compensation
Committee in good faith ("Fair Market Value").

                                      -2-
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      (d)   Duration of Options. Each Option shall be exercisable at such times
and subject to such terms and conditions as the Compensation Committee may
specify in the applicable option agreement, provided that the duration of any
such Option shall not exceed 10 years.

      (e)   Exercise of Option. Options may be exercised by delivery to the
Company of a written notice of exercise signed by the proper person or by any
other form of notice (including electronic notice) approved by the Compensation
Committee together with payment in full as specified in Section 5(f) for the
number of shares for which the Option is exercised.

      (f)   Payment Upon Exercise. Common Stock purchased upon the exercise of
an Option granted under the Plan shall be paid for as follows:

            (1)   in cash or by check, payable to the order of the Company;

            (2)   except as the Compensation Committee may, in its sole
discretion, otherwise provide in an option agreement, by (i) delivery of an
irrevocable and unconditional undertaking by a creditworthy broker to deliver
promptly to the Company sufficient funds to pay the exercise price and any
required tax withholding or (ii) delivery by the Participant to the Company of a
copy of irrevocable and unconditional instructions to a creditworthy broker to
deliver promptly to the Company cash or a check sufficient to pay the exercise
price and any required tax withholding;

            (3)   when the Common Stock is registered under the Exchange Act, by
delivery of shares of Common Stock owned by the Participant valued at their Fair
Market Value, provided (i) such method of payment is then permitted under
applicable law and (ii) such Common Stock, if acquired directly from the
Company, was owned by the Participant at least six months prior to such
delivery;

            (4)   to the extent permitted by applicable law and by the
Compensation Committee , in its sole discretion by payment of such other lawful
consideration as the Compensation Committee may determine; or

            (5)   by any combination of the above permitted forms of payment.

      (g)   Substitute Options. In connection with a merger or consolidation of
an entity with the Company or the acquisition by the Company of property or
stock of an entity, the Compensation Committee may grant Options in substitution
for any options or other stock or stock-based awards granted by such entity or
an affiliate thereof. Substitute Options may be granted on such terms as the
Compensation Committee deems appropriate in the circumstances, notwithstanding
any limitations on Options contained in the other sections of this Section 5 or
in Section 2.

6.    Restricted Stock

      (a)   Grants. The Compensation Committee may grant Awards entitling
recipients to acquire shares of Common Stock, subject to the right of the
Company to repurchase all or part of such shares at their issue price or other
stated or formula price (or to require forfeiture of such shares if issued at no
cost) from the recipient in the event that conditions specified by the
Compensation Committee in the applicable Award are not satisfied prior to the
end of the applicable restriction period or periods established by the
Compensation Committee for such Award (each, a "Restricted Stock Award").

                                      -3-
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      (b)   Terms and Conditions. The Compensation Committee shall determine the
terms and conditions of any such Restricted Stock Award, including the
conditions for repurchase (or forfeiture) and the issue price, if any.

      (c)   Stock Certificates. Any stock certificates issued in respect of a
Restricted Stock Award shall be registered in the name of the Participant and,
unless otherwise determined by the Compensation Committee , deposited by the
Participant, together with a stock power endorsed in blank, with the Company (or
its designee). At the expiration of the applicable restriction periods, the
Company (or such designee) shall deliver the certificates no longer subject to
such restrictions to the Participant or if the Participant has died, to the
beneficiary designated, in a manner determined by the Compensation Committee ,
by a Participant to receive amounts due or exercise rights of the Participant in
the event of the Participant's death (the "Designated Beneficiary"). In the
absence of an effective designation by a Participant, Designated Beneficiary
shall mean the Participant's estate.

7.    Stock Appreciation Rights.

      (a)   Nature of Stock Appreciation Rights. A Stock Appreciation Right, or
SAR, is an Award entitling the holder on exercise to receive an amount in cash
or Common Stock or a combination thereof (such form to be determined by the
Compensation Committee ) determined in whole or in part by reference to
appreciation, from and after the date of grant, in the fair market value of a
share of Common Stock. SARs may be based solely on appreciation in the fair
market value of Common Stock or on a comparison of such appreciation with some
other measure of market growth such as (but not limited to) appreciation in a
recognized market index. The date as of which such appreciation or other measure
is determined shall be the exercise date unless another date is specified by the
Compensation Committee in the SAR Award. The exercise price of a SAR shall in no
event be less than the Fair Market Value of the Common Stock on the date of
grant. The duration of a SAR shall not exceed 10 years.

      (b)   Grants. Stock Appreciation Rights may be granted in tandem with, or
independently of, Options granted under the Plan.

      (c)   Rules Applicable to Tandem Awards. When Stock Appreciation Rights
are expressly granted in tandem with Options, (i) the Stock Appreciation Right
will be exercisable only at such time or times, and to the extent, that the
related Option is exercisable and will be exercisable in accordance with the
procedure required for exercise of the related Option; (ii) the Stock
Appreciation Right will terminate and no longer be exercisable upon the
termination or exercise of the related Option, except that a Stock Appreciation
Right granted with respect to less than the full number of shares covered by an
Option will not be reduced until the number of shares as to which the related
Option has been exercised or has terminated exceeds the number of shares not
covered by the Stock Appreciation Right; (iii) the Option will terminate and no
longer be exercisable upon the exercise of the related Stock Appreciation Right;
and (iv) the Stock Appreciation Right will be transferable only with the related
Option.

      (d)   Exercise of Independent Stock Appreciation Rights. A Stock
Appreciation Right not expressly granted in tandem with an Option will become
exercisable at such time or times, and on such conditions, as the Compensation
Committee may specify in the SAR Award.

      (e)   Exercise. Any exercise of a Stock Appreciation Right must be in
writing, signed by the proper person and delivered or mailed to the Company,
accompanied by any other documents required by the Compensation Committee.

                                      -4-
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8.    Other Stock-Based Awards

      Other Awards of shares of Common Stock, and other Awards that are valued
in whole or in part by reference to, or are otherwise based on, shares of Common
Stock or other property, may be granted hereunder to Participants ("Other Stock
Unit Awards"), including without limitation Awards entitling recipients to
receive shares of Common Stock to be delivered in the future. Such Other Stock
Unit Awards shall also be available as a form of payment in the settlement of
other Awards granted under the Plan or as payment in lieu of compensation to
which a Participant is otherwise entitled. Other Stock Unit Awards may be paid
in shares of Common Stock or cash, as the Compensation Committee shall
determine. Subject to the provisions of the Plan, the Compensation Committee
shall determine the conditions of each Other Stock Unit Awards, including any
purchase price applicable thereto. At the time any Award is granted, the
Compensation Committee may provide that, at the time Common Stock would
otherwise be delivered pursuant to the Award, the Participant will instead
receive an instrument evidencing the Participant's right to future delivery of
the Common Stock.

9.    Adjustments for Changes in Common Stock and Certain Other Events

      (a)   Changes in Capitalization. In the event of any stock split, reverse
stock split, stock dividend, recapitalization, combination of shares,
reclassification of shares, spin-off or other similar change in capitalization
or event, or any distribution to holders of Common Stock other than an ordinary
cash dividend, (i) the number and class of securities available under this Plan,
(ii) the per-Participant limit set forth in Section 4(b), (iii) the number and
class of securities and exercise price per share subject to each outstanding
Option, (iv) the repurchase price per share subject to each outstanding
Restricted Stock Award, and (v) the share- and per-share-related provisions of
each outstanding Stock Appreciation Right and Other Stock Unit Award, shall be
appropriately adjusted by the Company (or substituted Awards may be made, if
applicable) to the extent the Compensation Committee shall determine, in good
faith, that such an adjustment (or substitution) is necessary and appropriate.
If this Section 9(a) applies and Section 9(c) also applies to any event, Section
9(c) shall be applicable to such event, and this Section 9(a) shall not be
applicable.

      (b)   Liquidation or Dissolution. In the event of a proposed liquidation
or dissolution of the Company, the Compensation Committee shall upon written
notice to the Participants provide that all then unexercised Options will (i)
become exercisable in full as of a specified time at least 10 business days
prior to the effective date of such liquidation or dissolution and (ii)
terminate effective upon such liquidation or dissolution, except to the extent
exercised before such effective date. The Compensation Committee may specify the
effect of a liquidation or dissolution on any Restricted Stock Award or other
Award granted under the Plan at the time of the grant of such Award.

      (c)   Reorganization Events

            (1)   Definition. A "Reorganization Event" shall mean: (a) any
                  merger or consolidation of the Company with or into another
                  entity as a result of which all of the Common Stock of the
                  Company is converted into or exchanged for the right to
                  receive cash, securities or other property or (b) any exchange
                  of all of the Common Stock of the Company for cash, securities
                  or other property pursuant to a share exchange transaction.

                                      -5-
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            (2)   Consequences of a Reorganization Event on Options. Upon the
                  occurrence of a Reorganization Event, or the execution by the
                  Company of any agreement with respect to a Reorganization
                  Event, the Compensation Committee shall provide that all
                  outstanding Options shall be assumed, or equivalent options
                  shall be substituted, by the acquiring or succeeding
                  corporation (or an affiliate thereof). For purposes hereof, an
                  Option shall be considered to be assumed if, following
                  consummation of the Reorganization Event, the Option confers
                  the right to purchase, for each share of Common Stock subject
                  to the Option immediately prior to the consummation of the
                  Reorganization Event, the consideration (whether cash,
                  securities or other property) received as a result of the
                  Reorganization Event by holders of Common Stock for each share
                  of Common Stock held immediately prior to the consummation of
                  the Reorganization Event (and if holders were offered a choice
                  of consideration, the type of consideration chosen by the
                  holders of a majority of the outstanding shares of Common
                  Stock); provided, however, that if the consideration received
                  as a result of the Reorganization Event is not solely common
                  stock of the acquiring or succeeding corporation (or an
                  affiliate thereof), the Company may, with the consent of the
                  acquiring or succeeding corporation, provide for the
                  consideration to be received upon the exercise of Options to
                  consist solely of common stock of the acquiring or succeeding
                  corporation (or an affiliate thereof) equivalent in fair
                  market value to the per share consideration received by
                  holders of outstanding shares of Common Stock as a result of
                  the Reorganization Event.

      Notwithstanding the foregoing, if the acquiring or succeeding corporation
(or an affiliate thereof) does not agree to assume, or substitute for, such
Options, then the Compensation Committee shall, upon written notice to the
Participants, provide that all then unexercised Options will become exercisable
in full as of a specified time prior to the Reorganization Event and will
terminate immediately prior to the consummation of such Reorganization Event,
except to the extent exercised by the Participants before the consummation of
such Reorganization Event; provided, however, that in the event of a
Reorganization Event under the terms of which holders of Common Stock will
receive upon consummation thereof a cash payment for each share of Common Stock
surrendered pursuant to such Reorganization Event (the "Acquisition Price"),
then the Compensation Committee may instead provide that all outstanding Options
shall terminate upon consummation of such Reorganization Event and that each
Participant shall receive, in exchange therefor, a cash payment equal to the
amount (if any) by which (A) the Acquisition Price multiplied by the number of
shares of Common Stock subject to such outstanding Options (whether or not then
exercisable), exceeds (B) the aggregate exercise price of such Options. To the
extent all or any portion of an Option becomes exercisable solely as a result of
the first sentence of this paragraph, upon exercise of such Option the
Participant shall receive shares subject to a right of repurchase by the Company
or its successor at the Option exercise price. Such repurchase right (1) shall
lapse at the same rate as the Option would have become exercisable under its
terms and (2) shall not apply to any shares subject to the Option that were
exercisable under its terms without regard to the first sentence of this
paragraph.

            (3)   Consequences of a Reorganization Event on Restricted Stock
                  Awards. Upon the occurrence of a Reorganization Event, the
                  repurchase and other rights of the Company under each
                  outstanding Restricted Stock Award shall inure to the benefit
                  of the Company's successor and shall apply to the cash,
                  securities or other property which the Common Stock was
                  converted into or exchanged for

                                      -6-
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                  pursuant to such Reorganization Event in the same manner and
                  to the same extent as they applied to the Common Stock subject
                  to such Restricted Stock Award.

            (4)   Consequences of a Reorganization Event on Other Awards. The
                  Compensation Committee shall specify the effect of a
                  Reorganization Event on any other Award granted under the Plan
                  at the time of the grant of such Award.

10.   General Provisions Applicable to Awards

      (a)   Transferability of Awards. Except as the Compensation Committee may
otherwise determine or provide in an Award, Awards shall not be sold, assigned,
transferred, pledged or otherwise encumbered by the person to whom they are
granted, either voluntarily or by operation of law, except by will or the laws
of descent and distribution, and, during the life of the Participant, shall be
exercisable only by the Participant. References to a Participant, to the extent
relevant in the context, shall include references to authorized transferees.

      (b)   Documentation. Each Award shall be evidenced in such form (written,
electronic or otherwise) as the Compensation Committee shall determine. Each
Award may contain terms and conditions in addition to those set forth in the
Plan.

      (c)   Compensation Committee Discretion. Except as otherwise provided by
the Plan, each Award may be made alone or in addition or in relation to any
other Award. The terms of each Award need not be identical, and the Compensation
Committee need not treat Participants uniformly.

      (d)   Termination of Status. The Compensation Committee shall, in the
applicable agreement with respect to an Award, determine the effect on an Award
of the disability, death, retirement, authorized leave of absence or other
change in the employment or other status of a Participant and the extent to
which, and the period during which, the Participant, the Participant's legal
representative, conservator, guardian or Designated Beneficiary may exercise
rights under the Award.

      (e)   Withholding. Each Participant shall pay to the Company, or make
provision satisfactory to the Compensation Committee for payment of, any taxes
required by law to be withheld in connection with Awards to such Participant no
later than the date of the event creating the tax liability. Except as the
Compensation Committee may otherwise provide in an Award, when the Common Stock
is registered under the Exchange Act, Participants may satisfy such tax
obligations in whole or in part by delivery of shares of Common Stock, including
shares retained from the Award creating the tax obligation, valued at their Fair
Market Value; provided, however, that the total tax withholding where stock is
being used to satisfy such tax obligations cannot exceed the Company's minimum
statutory withholding obligations (based on minimum statutory withholding rates
for federal and state tax purposes, including payroll taxes, that are applicable
to such supplemental taxable income). The Company may, to the extent permitted
by law, deduct any such tax obligations from any payment of any kind otherwise
due to a Participant.

      (f)   Amendment of Award. The Compensation Committee may amend, modify or
terminate any outstanding Award, including but not limited to, substituting
therefor another Award of the same or a different type, changing the date of
exercise or realization, and converting an Incentive Stock Option to a
Nonstatutory Stock Option, provided that the Participant's consent to such
action shall be required unless

                                      -7-
<PAGE>

the Compensation Committee determines that the action, taking into account any
related action, would not materially and adversely affect the Participant.

      (g)   Conditions on Delivery of Stock. The Company will not be obligated
to deliver any shares of Common Stock pursuant to the Plan or to remove
restrictions from shares previously delivered under the Plan until (i) all
conditions of the Award have been met or removed to the satisfaction of the
Company, (ii) in the opinion of the Company's counsel, all other legal matters
in connection with the issuance and delivery of such shares have been satisfied,
including any applicable securities laws and any applicable stock exchange or
stock market rules and regulations, and (iii) the Participant has executed and
delivered to the Company such representations or agreements as the Company may
consider appropriate to satisfy the requirements of any applicable laws, rules
or regulations.

      (h)   Acceleration. The Compensation Committee may at any time provide
that any Award shall become immediately exercisable in full or in part, free of
some or all restrictions or conditions, or otherwise realizable in full or in
part, as the case may be.

11.   Miscellaneous

      (a)   No Right To Employment or Other Status. No person shall have any
claim or right to be granted an Award, and the grant of an Award shall not be
construed as giving a Participant the right to continued employment or any other
relationship with the Company. The Company expressly reserves the right at any
time to dismiss or otherwise terminate its relationship with a Participant free
from any liability or claim under the Plan, except as expressly provided in the
applicable Award.

      (b)   No Rights As Stockholder. Subject to the provisions of the
applicable Award, no Participant or Designated Beneficiary shall have any rights
as a stockholder with respect to any shares of Common Stock to be distributed
with respect to an Award until becoming the record holder of such shares.
Notwithstanding the foregoing, in the event the Company effects a split of the
Common Stock by means of a stock dividend and the exercise price of and the
number of shares subject to such Option are adjusted as of the date of the
distribution of the dividend (rather than as of the record date for such
dividend), then an optionee who exercises an Option between the record date and
the distribution date for such stock dividend shall be entitled to receive, on
the distribution date, the stock dividend with respect to the shares of Common
Stock acquired upon such Option exercise, notwithstanding the fact that such
shares were not outstanding as of the close of business on the record date for
such stock dividend.

      (c)   Effective Date and Term of Plan. The Plan shall become effective on
the date on which it is adopted by the Compensation Committee . No Awards shall
be granted under the Plan after the completion of ten years from the earlier of
(i) the date on which the Plan was adopted by the Compensation Committee or (ii)
the date the Plan was approved by the Company's stockholders, but Awards
previously granted may extend beyond that date.

      (d)   Amendment of Plan. The Compensation Committee may amend, suspend or
terminate the Plan or any portion thereof at any time, provided that to the
extent required by Section 162(m), no Award granted to a Participant that is
intended to comply with Section 162(m) after the date of such amendment shall
become exercisable, realizable or vested, as applicable to such Award, unless
and until such amendment shall have been approved by the Company's stockholders
as required by Section 162(m) (including the vote required under Section
162(m)).

                                      -8-
<PAGE>

      (e)   Authorization of Sub-Plans. The Compensation Committee may from time
to time establish one or more sub-plans under the Plan for purposes of
satisfying applicable blue sky, securities or tax laws of various jurisdictions.
The Compensation Committee shall establish such sub-plans by adopting
supplements to this Plan containing (i) such limitations on the Compensation
Committee 's discretion under the Plan as the Compensation Committee deems
necessary or desirable or (ii) such additional terms and conditions not
otherwise inconsistent with the Plan as the Compensation Committee shall deem
necessary or desirable. All supplements adopted by the Compensation Committee
shall be deemed to be part of the Plan, but each supplement shall apply only to
Participants within the affected jurisdiction and the Company shall not be
required to provide copies of any supplement to Participants in any jurisdiction
which is not the subject of such supplement.

      (f)   Governing Law. The provisions of the Plan and all Awards made
hereunder shall be governed by and interpreted in accordance with the laws of
the State of Delaware, without regard to any applicable conflicts of law.

                                      * * *

                        APPROVED BY THE BOARD OF DIRECTORS ON FEBRUARY 20, 2004

                        APPROVED BY THE STOCKHOLDERS ON [_____], 2004

                                      -9-<PAGE>

                                                                  EXHIBIT 10.33

                                   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..................................................................................       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
   2.9      Standstill Agreement......................................................................       5

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      Material Adverse Effect...................................................................       7
   4.8      Legal Opinion.............................................................................       7
   4.9      Absence of Litigation.....................................................................       7

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

   5.1      Representations and Warranties............................................................       7
   5.2      Qualifications............................................................................       7
   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......................................................................       8
   6.7      Notices...................................................................................       9
   6.8      Finder's Fees.............................................................................      10
   6.9      Expenses..................................................................................      10
   6.10     Attorneys' Fees...........................................................................      10
   6.11     Amendments and Waivers....................................................................      10
   6.12     Severability..............................................................................      10
   6.13     Indemnification...........................................................................      10
   6.14     Remedies..................................................................................      11
</TABLE>

EXHIBIT

Exhibit A Form of Opinion of Hale and Dorr LLP

                                       ii
<PAGE>

                                    PAR VALUE
                            STOCK PURCHASE AGREEMENT

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

                                       2

<PAGE>

            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, 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,

                                        3
<PAGE>

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 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

                                        4
<PAGE>

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.

            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 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 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

                                        7
<PAGE>

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.

            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.

                                       8
<PAGE>

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

                                        9
<PAGE>

            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 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

                                       10
<PAGE>

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.

                                       11
<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:_____________________                          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 Par Value 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 Par Value
      Stock Purchase Agreement, the issue and sale of the Shares, the compliance
      by the Company with all of the provisions of the Par Value 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
      Par Value 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 Par Value 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.

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

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