Document:

<PAGE>
                                                                 Exhibit 10.18.1

     Confidential Materials omitted and filed separately with the Securities
              and Exchange Commission. Asterisks denote omissions.

                                    AGREEMENT

This agreement is made by and between

IDENIX PHARMACEUTICALS, INC.  a corporation organized and existing under
the laws of the State of Delaware, having its principal offices located at
125 CambridgePark Drive, Cambridge, MA 02140 represented by its legal
representative Mr. Jean-Pierre Sommadossi (hereinafter referred to as
"Idenix")

                                       and

IDENIX SARL, a corporation organized and existing under the laws of France,
having registered offices in Immueuble "La Vigie" 170 rue Leon Blum, 34000
Montpellier, France, represented by its legal representative Mr. Jean-Marc
Allaire (hereinafter referred to as "Idenix SARL")

                                       and

UNIVERSITA' DELGI STUDI DI CAGLIARI, having a principal place of business at
Cittadella Universitaria, SS 554 KM 4.5, 09133 Monserrato, Cagliari, Italy,
represented by the Chancellor Prof. Pasquale Mistretta (hereinafter referred to
as the "University")

                                     whereas

a) on 4th of January 1999 the University and Idenix SARL, this last on behalf
and for the benefit of Idenix, entered into an agreement entitled "Co-operative
Antiviral Research Activity Agreement" (hereinafter referred to as "Co-operative
Agreement") aimed at performing a joined research activity in the antiviral
substances field;

b) in accordance with the Co-operative Agreement, on 14 December 2000 the
University and Idenix entered into a license agreement (hereinafter "License
Agreement") according to which the University grants to Idenix the exclusive
license for the exploitation, whether direct or indirect, of the results
obtained and that will be obtained from the performance of the activity
indicated under the previous point;

c) the above mentioned Co-operative Agreement and License Agreement have
been initially amended on 10 April 2002;

d) said agreements have been further amended with the deed undersigned by the
parties on May 8, 2003, also in consideration of the transactions at that time
pending between Idenix and Novartis Pharma AG for the acquisition, by this last,
of the majority of the shareholding of Idenix (the amendments indicated in this
point have become effective on May 8, 2003, following
<PAGE>
to the occurred execution of a "Development, License and Commercialization
Agreement" and of a "Manufacturing and Supply Agreement" by and among Idenix,
Idenix (Cayman) Limited and Novartis);

e) in compliance with the provision set forth under art. 9.1 of the
Co-operative Agreement, art. 3 of the License Agreement states that Idenix
shall pay to the University earned royalties at a rate equal to

      (i)   [**]% of Net Sales in case of direct exploitation of the results
            obtained from the performance of the research activity in accordance
            with Co-operative Agreement;

      (ii)  [**]% of Sublicense Payments or Royalties in case of
            sublicense of the Intellectual Property Rights

now, therefore, in consideration of the premises and the mutual covenants herein
contained, Idenix, Idenix SARL and the University agree as follows:

1.  The premises shall represent an integral and substantial part of this
agreement.

2.  Also in consideration of the relevance of the results that the parties
are obtaining from the joined research carried out, the royalty rate
payable by Idenix or any of its subsidiaries indicated under art. 9.1 of
the Co-operative Agreement and art. 3 of the License Agreement shall be
increased from [**]% to [**]%.

The parties hereby furthermore agree that such an increased royalty rate shall
apply to technology and other property that becomes initially subject to patent
applications (including US provisional applications) which constitute the parent
case filed on or after 14 May 2003.

For the purposes of this point 2, the date of the first filing of each parent
case patent application shall be considered (not the date of the corresponding
foreign extensions).

The parties hereby agree that the rate of royalty will be exclusively determined
by reference to the date of filing of the parent application.

3. The percentage defined under previous point 2 shall be calculated on Net
Sales whether performed directly by Idenix or by a sublicense. As regards to
patent applications, including US provisional patent applications, filed before
14 May 2003, from the execution date of the present agreement, the [**]% royalty
shall as well be calculated on Net Sales whether performed directly by Idenix or
by a sublicense. For the purpose of this section 3, the definition of "Net
Sales" provided under section 1.10 of the License Agreement shall apply.

3.1. Idenix shall provide the University of Cagliari with the right of a yearly
audit to check the correct undertakings and fulfillments of any obligations of
the Cooperative Agreement, of the License and relevant amendments.

4. The present document represents an amendment to art. 9.1 of the Co-operative
Agreement and to art. 3 of the License Agreement that, with reference to any
other provision, shall entirely apply in the current version (and with the
possible amendments that might be brought in the future) to the relationships
existing among the parties (and with the possible amendments brought in the
<PAGE>
future). The parties however agree that in any case in which it is provided to
the University the right to grant non exclusive licenses, this has to be
understood both as non exclusive and exclusive license right.

5. The parties hereby agree that the patents filed by or on behalf of Idenix or
Novartis in order to protect the results of the activity performed in pursuance
of the Cooperative Agreement, are therefore deemed subject to the terms of the
License Agreement and shall be included in the Annex to the License Agreement. A
copy of Annex A as it is currently constituted is attached hereto and made a
part of the License Agreement. The Annex A attached hereto supercedes in its
entirety the Annex A referred to in the amendment dated 10 April 2002 of the
License Agreement and Co-operative Agreement. In the future, Idenix will provide
to include in such Annex the new patent applications concerning results obtained
within the Cooperative Activity, providing the University with a quarterly
communication of such Annex including the new patent applications which updated
Annex A will be deemed to be agreed to by the University.

In the Annex A attached hereto are not included the following patents: US patent
application No. 60/626184 (Sommadossi) and US patent application No. 60/326192
(Sommadossi) that were un-correctly inserted in previous Annex A of the License
Agreement, since they are not part of the Co-operative Antiviral Research
Activity Agreement due to the fact that Professor Paolo LA Colla did not
contributed, even partially, to the achievement of the relevant inventions.

6. In order to clarify the provisions set forth in the agreement undersigned by
the parties on 8 May 2003 mentioned in the premises, the parties recognize and
accept that the rights and the obligations, stated upon the Dipartimento di
Scienze e Tecnologie Biomediche by the Co-operative Agreement and by the License
Agreement (as finally amended on 8 may 2003) shall be considered exclusively
referred to the Sezione di Microbiologia e Virologia Generale e Biotecnologie
Microbioche, directed by Prof. La Colla.

7. The parties hereto agree that the initial term of the Co-operative Agreement,
as amended, shall expire on January 22, 2007 and that the Co-operative Agreement
may be renewed for additional four year periods by mutual consent of the parties
expressed in writing at least 30 days prior to the expiration of the then
current term.

8. This agreement is drafted in English, and for the sole purpose of allowing
the University to better comprehend the relevant content, in Italian, being
understand that in case of contrasts between the English version and the Italian
version, the English one shall prevail.

    June 30, 2004
--------------------------------------
    Date

   /s/Jean Pierre Sommadossi
--------------------------------------
name and appointment

IDENIX PHARMACEUTICALS, INC.

   /s/Jean Pierre Sommadossi
--------------------------------------
name and appointment
<PAGE>
IDENIX SARL

/s/ Pasquale Mistietia
--------------------------------------
Prof. Pasquale Mistietia

/s/ Pavlo LaColla
--------------------------------------
Prof. Pavlo LaColla
<PAGE>
                                    EXHIBIT A

<TABLE>
<CAPTION>
                                                                            PATENT /       ISSUE /
TITLE                                                 FILING   SERIAL       PUBLICATION    PUB.
& DOCKET NAME    COUNTRY   OWNER        INVENTORS     DATE     NUMBER       NUMBER         DATE        STATUS
-------------    -------   -----        ---------     ----     ------       ------         ----        ------
<S>              <C>       <C>          <C>           <C>      <C>          <C>            <C>         <C>
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
                                        P.
                                        LaColla,
IDX1006          U.S.      USC IDXINC   JP.           5/26/01  09/863,816   2003/0060400   03/27/03    Allowed
                                        Sommadossi
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
                                        P.
                                        LaColla,
IDX1006 CON 3    U.S.      USC IDXINC   JP.           6/20/03  10/602,692   2004/0097462   05/20/04    Published
                                        Sommadossi
                                        P.
                                        LaColla,
IDX1006 DIV      U.S.      USC IDXINC   JP.           6/20/03  10/602,693   2004/0063622   04/01/04    Published
                                        Sommadossi
[**]             [**]         [**]      [**]          [**]     [**]         [**]           [**]        [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]         [**]                       [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]         [**]                       [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]         [**]                       [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]         [**]                       [**]
[**]             [**]         [**]      [**]          [**]     [**]         [**]                       [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]         [**]                       [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
</TABLE>
<PAGE>
<TABLE>
<CAPTION>
                                                                            PATENT /       ISSUE /
TITLE                                                 FILING   SERIAL       PUBLICATION    PUB.
& DOCKET NAME    COUNTRY   OWNER        INVENTORS     DATE     NUMBER       NUMBER         DATE        STATUS
-------------    -------   -----        ---------     ----     ------       ------         ----        ------
<S>              <C>       <C>          <C>           <C>      <C>          <C>            <C>         <C>
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]         [**]           [**]        [**]
                                        P.
                              USC       LaColla,
IDX1007CON1      U.S.        IDXINC     JP.           6/20/03  10/602,691   2004/0097461   05/20/04    Published
                                        Sommadossi
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]         [**]           [**]        [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
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[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
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[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]         [**]                       [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]         [**]                       [**]
[**]             [**]         [**]      [**]          [**]     [**]         [**]                       [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]         [**]                       [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]         [**]                       [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]         [**]           [**]        [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
</TABLE>
<PAGE>
<TABLE>
<CAPTION>
                                                                            PATENT /       ISSUE /
TITLE                                                 FILING   SERIAL       PUBLICATION    PUB.
& DOCKET NAME    COUNTRY   OWNER        INVENTORS     DATE     NUMBER       NUMBER         DATE        STATUS
-------------    -------   -----        ---------     ----     ------       ------         ----        ------
<S>              <C>       <C>          <C>           <C>      <C>          <C>            <C>         <C>
[**]             [**]         [**]      [**]          [**]     [**]         [**]           [**]        [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]         [**]                       [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]         [**]           [**]        [**]
                                        J-P
                                        Sommadossi
IDX1009CON1      US CON    USC IDXINC   P LaColla     1/24/03  10/350,772   2003/0225114   12/04/03    Published
                                        M Artico
                                        M Bryant
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]         [**]           [**]        [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
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[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
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[**]             [**]         [**]      [**]          [**]     [**]         [**]           [**]        [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
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[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
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[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
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[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]         [**]           [**]        [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
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[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
</TABLE>
<PAGE>
<TABLE>
<CAPTION>
                                                                            PATENT /       ISSUE /
TITLE                                                 FILING   SERIAL       PUBLICATION    PUB.
& DOCKET NAME    COUNTRY   OWNER        INVENTORS     DATE     NUMBER       NUMBER         DATE        STATUS
-------------    -------   -----        ---------     ----     ------       ------         ----        ------
<S>              <C>       <C>          <C>           <C>      <C>          <C>            <C>         <C>
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]         [**]           [**]        [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
[**]             [**]         [**]      [**]          [**]     [**]                                    [**]
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[**]             [**]         [**]      [**]          [**]     [**]
[**]             [**]         [**]      [**]          [**]     [**]

IDX 1031
2'-C-Methyl-3'-
O-L-Valine
Ester                                   JP
Ribofuranosyl    U.S.         USC.      Sommadossi    6/27/03  10/607,909   2004/0077587   04/22/04    Published
Cytidine for                 IDXINC     P La
Treatment of                            Colla
Flaviviridae
Infections
(NM283)

[**]             [**]         [**]      [**]          [**]     [**]         [**]           [**]        [**]
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</TABLE>

        [**]
        [**]
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        [**]<PAGE>
            COLLABORATIVE ACTIVITIES AT LABORATORIES IN MACCHIAREDDU

     1.   Within the newly leased facility (total 850 sq. m), 250 sq. m of space
          (ex Stabulario) will be exclusively made available for use by Idenix
          employees. Idenix employees will also enjoy of common area (total 350
          sq. m) use similar to all other University employees occupying the
          building. Idenix will contribute up to E 54,000 per year during 3
          years to the cost of University of Cagliari for renting this facility
          subject to the conditions noted in paragraph 4 below.

     2.   Idenix will restore and equip at its own expenses its laboratory space
          at Macchiareddu, that will include a BL3 laboratory.

          a.   Idenix will have priority for use of the BL3.

          b.   Pr. LaColla will be administratively responsible for the BL3
               physical plant.

          c.   Idenix equipment at Macchiareddu will become property of the
               University of Cagliari when the Cooperative Agreement between the
               University and Idenix expires or is terminated.

     3.   Pr. LaColla will have access to all Idenix compounds and scientific
          data generated as a follow up of cooperative activities previously
          involving the Cagliari Team and will be named as a co-inventor on any
          Intellectual Property to which he has contributed concerned with the
          developments of such research activities. Pr. LaColla will be named as
          a co-inventor on Intellectual Property, for which he meets the
          criteria as a contributor to the invention, that will be generated by
          the Idenix team in Cagliari or the Cagliari team.

     4.   Pr. LaColla will work with the Director of Virology (see below) to
          establish the budget of the University of Cagliari - Idenix
          Cooperative Laboratory (Cooperative Laboratory). Idenix will provide
          the following (in 1000s of Euro) for the Cooperative Laboratory. As
          set forth below, the total 2004 budget for the Cooperative Laboratory
          will be 1.363MM Euro.

<Table>
<Caption>
                                             Idenix         UNICA
                                             ------         -----
<S>                                          <C>            <C>
Running Cost                                 275
Salaries and bonus pool                      560
Travel                                       20
Capex (to be agreed between Pr La Colla
   and D. Shlaes)                            208
Rent (Macchiareddu)                          54
Unrestricted Grant                                          250
</Table>

          CAPEX will be agreed between Pr. LaColla and D. Shlaes.

          Idenix will provide the University of Cagliari with an unrestricted
          grant of E 250,000 per year during 3 years [2004 - 2006], which can be
          used for travel, salaries, running costs, etc. Idenix will pay the
          unrestricted grant on a monthly basis. However, the obligation of
          Idenix to make such payment and other payments relating to the
          operation of the Cooperative Laboratory is conditioned upon the
          Co-operative Antiviral Research Activity Agreement dated January 4,
          1999, by and between Dipartimento di Biologia Sperimentale "Bernardo
          Loddo" dell Universita di Cagliari and Idenix SARL, as amended,
          remaining in full force and effect throughout such period.

     5.   Pr. LaColla will remain the Director of the Cooperative Laboratory and
          will continue to provide scientific guidance to the Idenix team.

     6.   Idenix has identified and employ a senior Director of Virology for the
          Idenix Team in Cagliari. The roles of the Director of Virology vis a
          vis Idenix, the Cooperative Laboratory and Pr. LaColla are described
          below.

GENERAL MISSION

In accordance with the Idenix' R&D objectives defined by the Exec V.P. for R&D,
Director, Virology, in concert with the Cooperative Laboratory Director, will
propose and implement Idenix' strategy for anti-viral screening within the
facilities at the University of Cagliari utilized by the Idenix employees
and/or through academic collaborations or contract organizations.

Director, Virology, in concert with the Cooperative Laboratory Director, will
be responsible for budget proposal and implementation of the approved budget
for the Idenix team in Cagliari (head count, scientific equipment, consumables,
travel, training, scientific consultancies).

Director, Virology will be responsible for Idenix Human Resources management
concerning yearly appraisal, salary and rewarding proposal, recruitment
proposal and selection.

CONTEXT AND STAKES OF THE POSITION

Director, Virology will be based in Cagliari within Idenix SARL.

Internal relations
------------------

Director,, Virology reports to the Executive V.P. for R&D, who is located in US
within Idenix Pharmaceuticals Inc. (Mass). For all budget and HR questions in
EU, he follows Idenix SARL procedures.

External relations
------------------

Director, Virology will be responsible for the supervision of the Idenix
facilities located on University of Cagliari premises. Supervision will be
assumed in respect of the input and role of the Cooperative Laboratory Director
as defined in the collaborative convention signed with the University of
Cagliari.

The Cooperative Laboratory Director is responsible of the implementation of the
research program and of the budget and means allocated to the Cooperative
Laboratory by the two parties, as defined once a year during the steering
committee meeting. He is responsible for obtaining agreement from the two
parties before any publication and communication. He has a role of proposition
for matters related to rewarding proposal, travel and training for Cooperative
Laboratory personnel. He is responsible for implementing hygiene and security
procedures towards the lab personnel in accordance with the University policy.

Director, Virology will supervise for the benefit of Idenix the implementation
of the research program and of the budget and means allocated by Idenix to the
Idenix Team in the Cooperative Laboratory. This will be done in respect of the
global objectives agreed on by the two parties once a year during the steering
committee meeting. In addition Director, Virology, will be directly responsible
for Human Resources management of Idenix personnel concerning commitment to
Idenex objectives, yearly appraisal, salary and rewarding proposal, travel and
training.

In case of need to modify in a significant way the research program or the
means allocated by Idenix to the Cooperative Laboratory, the Director, Virology
will inform the Idenix members of the steering committee and the President of
the steering committee, the latter may call for an extraordinary meeting for
redefining with the two parties the cooperative lab research program.

--------------------------------------------------------------------------------

Heretofore agreed this 22 day of March, 2004:

/s/ Jean-Pierre Sommadossi                  /s/ Paolo LaColla
------------------------------------        -------------------------
Jean-Pierre Sommadossi, Ph.D.               Prof. Paolo LaColla
Gerant                                      Dep. of Biomedical Sciences and
Idenix SARL                                 Technologies, University of Cagliari
Montpellier, France                         Cagliari, Sardinia, IT
<PAGE>
Cagliari, June 30, 2004

Universita degli Studi di Cagliari
Cittadella Universitaria
SS 554 Km 4,5
09133 Monserrato (CA)

To the attention of Magnifico Professor Pasquale Mistretta

Re:  Collaborative Activities at Laboratories in Macchiareddu

Magnifico Rettore,

Relating to the agreement "Collaborative Activities at Laboratories in
Macchiareddu" dated March 22, 2004 it is understood that there will be a partial
modification to article 4 of the agreement (Rent Macchiareddu) that provides for
Idenix to be responsible for rent in an annual amount of 90,000 euro + tax

Cordaialment

IDENIX SARL

Jean-Pierre Sommadossi

/s/ Jean-Pierre Sommadossi

Agreed

Universita degli Studia di Cagliari

Magnifico Rettore
Prof. Pasquale Mistretta

/s/ Prof. Pasquale Mistretta

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