<Page>

                                                                   Exhibit 10.18


                      FIRST AMENDMENT TO SECURITY AGREEMENT


     This First Amendment to Security Agreement (this "Amendment") is dated as
of October 15, 2003, by and between Dyax Corp. (the "Debtor") and Genzyme
Corporation (the "Secured Party").

     The following sets forth background information relevant to this Amendment:

     A.     Reference is hereby made to that certain Security Agreement dated as
of May 31, 2002, by the Debtor as debtor and the Secured Party as secured party
(the "Original Security Agreement"). Capitalized terms used in this Amendment
that are not otherwise defined herein shall have the meaning set forth in the
Original Security Agreement.

     B.     Debtor exercised the Surgical Product Option in Section 3.2 of the
Collaboration Agreement that is mentioned in Section 2(b) of the Original
Security Agreement, and accordingly, as provided in Section 2(b) of the Original
Security Agreement, the Additional Collateral was to become Collateral within
the meaning of the Original Security Agreement. Notwithstanding the foregoing,
Debtor has requested, and Secured Party has agreed, that Debtor may transfer to
a third party all of Debtor's right, title and interest in Biotage, Inc. and
therefore, such interests shall be no longer available to serve as Collateral.

     C.     Because of Debtor's transfer of the Shares of Biotage, Debtor and
Secured Party have agreed that Debtor shall grant to Secured Party the new
collateral pursuant to this Amendment.

     D.     Debtor and Secured Party wish to amend the Original Security
Agreement to, among other things, address the foregoing.

     NOW, THEREFORE, in consideration of the foregoing and other good and
valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, Debtor and Secured Party hereby agree as follows:

     1.     AMENDMENTS TO SECURITY AGREEMENT.

            (a)     The term "Program" is deleted from Section 1 of the Original
     Security Agreement and is replaced by the following:

                    "'Program' means the Debtor's research, development,
            production and sales relating to human plasma Kallikrein inhibitors,
            including that certain 58-amino acid polypeptide human plasma
            Kallikrein inhibitor known as "DX-88" (formerly known as EPI-KAL-2)
            together with all analogs, homologs, derivatives and improvements
            related thereto; but specifically excludes the Debtor's research,
            development, production and sales relating to the use of human
            plasma Kallikrein inhibitors in connection with the reduction of
            blood loss and other effects of systemic inflammatory responses
            during surgery."

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            (b)     The following new term "Ladner Patents" is added to Section
     1 of the Original Security Agreement:

                    "'Ladner Patents' shall mean the patents and patent
            applications, owned or controlled by Debtor, listed on SCHEDULE C
            attached hereto and any and all foreign counterparts and any and all
            divisionals, continuations or continuations-in-part, reissues,
            renewals, reexaminations, extensions, substitutions, supplementary
            protection certificates and additions thereof."

            (c)     The following new term "Ladner Patent License Agreements" is
     added to Section 1 of the Original Security Agreement:

                    "'Ladner Patent License Agreements' shall mean license and
            similar agreements between Debtor and third parties, whether now
            existing or hereafter arising, under which (1) Debtor grants a
            license to such third parties under the Ladner Patents to practice
            phage display (the "Ladner Technology") to discover human
            antibodies, peptides and proteins; (2) the consideration received by
            Debtor is solely based upon the license granted to such third party
            to practice Ladner Technology; and (3) Debtor transfers no materials
            or technology (other than the Ladner Technology) to such third
            party, and Debtor performs no research or other services on behalf
            of such third party. A list of current Dyax Patent License
            Agreements is attached as Schedule D."

            (d)     The following new term "Records" is added to Section 1 of
     the Original Security Agreement:

                    "'Records' shall mean books, records, computer software,
            computer printouts, customer lists, manuals and similar items."

            (e)     Section 2 of the Original Security Agreement is deleted in
     its entirety and the following shall be substituted therefore:

            "2.     COLLATERAL. To secure the full and prompt payment and
            performance of the Obligations (as defined below), the Debtor grants
            the Secured Party a continuing security interest in all of the
            Debtor's right, title and interest in and to the following property,
            whether now owned or existing or hereafter acquired or arising:

                    (a)    all tangible and intangible personal property arising
                    from, related to or used in connection with the Program,
                    including, but not limited to, the following: (i) all goods,
                    accounts, and general intangibles (as those terms are
                    defined in the UCC), (ii) all Intellectual Property
                    including, but not limited to, the patents issued or
                    assigned to and all patent applications made by the Debtor
                    relating to human plasma Kallikrein inhibitors, including
                    that certain 58-amino acid polypeptide human plasma
                    Kallikrein inhibitor known as "DX-88" (formerly known as
                    EPI-KAL-2), including, without limitation, the patents,
                    patent applications and exclusive licenses listed on
                    SCHEDULE A hereto, but specifically excluding Intellectual
                    Property relating to the use of human plasma Kallikrein
                    inhibitors, including DX-88, in connection with the
                    reduction of blood loss and other effects of systemic
                    inflammatory

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                    responses during surgery; (iii) all infringement claims with
                    regard to such Intellectual Property; and (iv) all rights to
                    revenue, income, profits, royalties, damages, licenses or
                    other rights related to such Intellectual Property;

                           (b)    (i) all accounts (as defined in the UCC)
                    arising from or related to the Ladner Patent License
                    Agreements and (ii) all rights to revenue, income, profits,
                    royalties, damages, licenses and other rights related to the
                    Ladner Patent License Agreements; and

                           (c)    all Records and all proceeds (as defined by
                    the UCC) and products of the foregoing set forth in
                    subsections (a) and (b) above (collectively, the
                    "COLLATERAL").

            Notwithstanding anything to the contrary, the Collateral shall not
            include any equipment."

            (f)     The Secured Party consents to Debtor's transfer of Biotage,
     Inc. Accordingly, (i) the fourth sentence of Section 4(a) of the Security
     Agreement is hereby deleted; (ii) Section 7(e) of the Original Security
     Agreement is hereby deleted; and (iii) references in the Original Security
     Agreement to the "Additional Collateral" shall be of no further force or
     effect.

            (g)     All references in the Security Agreement, as amended by this
     Amendment, to Schedule A, Schedule B, Schedule C and Schedule D, shall mean
     the Schedule A, Schedule B, Schedule C and Schedule D attached hereto.

            (h)     All references in the Note to the Security Agreement shall
     mean the Original Security Agreement, as amended by this Amendment.

     2.     REPRESENTATIONS AND WARRANTIES. Debtor hereby ratifies and confirms
all of the representations and warranties in the Original Security Agreement as
of the date of this Amendment and agrees that such representations and
warranties apply to the Debtor Collateral after giving effect to this Amendment.
Debtor hereby represents and warrants that as of the date of this Amendment, the
Collateral is free from any right or claim of any person or any Lien.

     3.     ADDITIONAL COVENANTS. The following shall be added as Section 7B. of
the Original Security Agreement:

            "7B.    NOTIFICATION TO ACCOUNT DEBTORS AND OTHER PERSONS OBLIGATED
     ON COLLATERAL. If an Event of Default shall have occurred and be
     continuing, the Debtor shall, at the request and option of the Secured
     Party, notify account debtors and other persons obligated on any of the
     Collateral of the security interest of the Secured Party in any account,
     general intangible, or other Collateral and that payment thereof is to be
     made directly to the Secured Party or to any financial institution
     designated by the Secured Party as the Secured Party's agent therefor, and
     the Secured Party may itself, if an Event of Default shall have occurred
     and be continuing, following five (5) business days written notice to
     Debtor, so notify account debtors and other persons obligated on
     Collateral. After the making of such a request or the giving of any such
     notification, the Debtor shall hold any proceeds of collection of accounts,
     general intangibles, and other Collateral received by the Debtor as trustee
     for the Secured Party without commingling

                                      - 3 -
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     the same with other funds of the Debtor and shall turn the same over to the
     Secured Party in the identical form received, together with any necessary
     endorsements or assignments. The Secured Party shall apply the proceeds of
     collection of accounts, general intangibles, and other Collateral received
     by the Secured Party to the Obligations, such proceeds to be immediately
     credited after final payment in cash or other immediately available funds
     for the items giving rise to them. Debtor irrevocably directs and requires
     all licensees and account debtors to honor Secured Party's request for
     direct payment and comply with any such request, notwithstanding any
     directions or instructions to the contrary that may be given by Debtor and
     agrees that the compliance by such licensee or account debtor with the
     provisions of this Section shall not be deemed a violation of such party's
     contractual agreements with Debtor."

     4.     SECURITY AGREEMENT; RATIFIED AND CONFIRMED. From and after the
effective date of this Amendment, the term "Security Agreement" shall mean the
Original Security Agreement, as amended by this Amendment. Except as expressly
amended hereby, the Security Agreement shall remain in full force and effect and
is hereby ratified and confirmed.

     5.     COUNTERPARTS. This Amendment may be executed in one or more
counterparts, each of which shall be deemed an original and all of which shall
constitute one and the same instrument.

     6.     CAPTIONS. The captions are headings in this Amendment are for
convenience only and in no way define, limit or describe the scope or intent of
any provisions or sections hereof.

     7.     GOVERNING LAW. This Amendment shall be governed by the laws of the
Commonwealth of Massachusetts.

     8.     SECURED PARTY'S EXPENSES. The Debtor agrees that it shall pay to the
Secured Party within twenty (20) days of the Secured Party's request, all legal
fees and expenses incurred by the Secured Party in connection with this
Amendment up to a maximum of $2,500.00.

     IN WITNESS WHEREOF, the parties have executed this Amendment under seal as
of the date first above written.

GENZYME CORPORATION                            DYAX CORP.


By: /s/ Richard Douglas                        By: /s/ Henry E. Blair
   -------------------------------------          ------------------------------
   Name: Richard Douglas                          Name: Henry E. Blair
   Title: SVP of Corporate Development            Title: Chairman & CEO

                                      - 4 -
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                                   SCHEDULE A

                                  DX-88 PATENTS

<Table>
<Caption>
                         APPLICATION
    COUNTRY                NUMBER              FILING DATE               STATUS
------------------  ---------------------  -------------------  ------------------------
<S>                 <C>                    <C>                  <C>
     U.S.                08/179,964              1/11/94        ABANDONED IN FAVOR OF US
                                                                        08/208,264

   U.S. CIP              08/208,264              3/10/94              U.S. 6,057,287
                                                                      ISSUED 5/20/00

   U.S. CIP              08/676,125              9/25/96             U.S. 5,795,865
                                                                      ISSUED 8/18/98

U.S. CIP - DIV           09/136,012              8/17/98              U.S. 5,994,125
                                                                     ISSUED 11/30/99

U.S. CIP - DIV           09/421,097             10/19/99              U.S. 6,333,402
                                                                     ISSUED 12/25/01

U.S. CIP - DIV           10/016,329             10/26/01                 PENDING

    CANADA               2,180,950               1/11/95                 PENDING

      EPO               95/909223.0              1/11/95                 PENDING
                    PUBLISHED EP0739355    PUB. DATE: 10/30/96

    JAPAN               518726/1995              1/11/95                 PENDING
                    PUBLISHED JP9-511131   PUB. DATE: 11/11/97

     PCT                  US95/00299             1/11/95              NATIONAL PHASE
                     PUBLISHED WO95/21601  PUB. DATE: 8/17/95
</Table>

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                                   SCHEDULE B


Existing Indebtedness:
---------------------

Capital Lease Obligations
-------------------------

Master Lease Agreement between General Electric Capital Corporation and Dyax
Corp. (indebtedness under lease line as of September 30, 2003 of $2,559)

Master Lease Agreement between Transamerica Business Credit Corporation and
Dyax Corp. (indebtedness under lease line as of September 30, 2003 of
$534,965)

Master Lease Agreement between Dyax SA and Locabel (indebtedness under lease
line as of September 30, 2003 of Eur 1,204,521)

Master Lease Agreement between Access National and Biotage Inc. (indebtedness
under lease line as of September 30, 2003 of $167,667)

Capital Lease for equipment in the Netherlands (indebtedness under lease line
as of September 30, 2003 of Eur 94,121)

Various capital leases with Dell Computer Leasing (indebtedness under lease
lines as of September 30, 2003 of $32,948)

Other Debt
----------

Loan obligation of Massachusetts Institute of Technology to Dyax Corp. under
Lease Agreement to loan up to $35 per rentable square foot to fund portion of
Tenant's Work (indebtedness under loan as of September 30, 2003 of $2,179,747)

Promissory note to Virginia National Bank given by Biotage Real Estate LLC to
Industrial Development Authority of Albermarle County, Virginia to fund land
acquisition and construction of new facility for Biotage, Inc. (indebtedness
under note as of September 30, 2003 of $4,232,087)

Promissory note to Virginia National Bank to fund the purchase of fixed
assets (indebtedness under note as of September 30, 2003 of $331,083)

Existing Liens:
--------------

Letter of Credit for benefit of Massachusetts Institute of Technology
($4,279,932) to secure obligations under Lease Agreement for 300 Technology
Square

All Equipment acquired pursuant to Master Lease Agreements and other lease
agreements under 6 Existing Capital Lease Obligations listed above

Real Estate located in Albermarle County, Virginia to secure obligations
under Promissory Note held by Virginia National Bank


                                      - 6 -
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                                   SCHEDULE C

                                 LADNER PATENTS

<Table>
<Caption>
             APPLICATION/                      PATENT      ISSUE
COUNTRY        PATENT NO.        FILING DATE   NO.         DATE/STATUS
-----------  ------------------  ------------  ----------  --------------
<S>          <C>                 <C>           <C>         <C>
US           07/664,989          3/1/91        5,223,409   6/29/93

US           08/009,319          1/26/93       5,403,484   4/4/95

US           08/057,667          6/18/93       5,571,698   11/5/96

US           08/415,922          4/3/95        5,837,500   11/17/98

US           Div. of 08/415,922  02/14/01                  Pending

US           Div of 08/415,922   03/05/01                  Pending

US           Div of 08/415,922   06/29/01                  Pending

US           Div of 08/415,922   06/29/01                  Pending

US           Div of 08/415,922   04/22/02                  Pending

US           Div of 08/415,922   04/22/02                  Pending

PCT          US89/03731          9/1/89                    National Phase
             W09002809 pub       3/22/90

EPO          96/112867.5         9/1/89                    Pending
             0 768 377 pub       4/16/97 pub

EPO          00/106289.2         9/1/89                    Pending
             1 026 240 pub       9/8/00 pub

Japan        89510087            9/1/89                    Pending
             JP4502700 (pub)     5/21/92

Canada       610,176             9/1/89        1,340,288   1/27/99

Ireland      IR89/2834           9/4/89                    Pending

Israel       91501               9/1/89        91501       6/11/98

Israel       3 Divisionals       5/29/97                   Pending

PCT          US92/01456          2/27/92                   National Phase
             W09215677 (pub)     9/17/92

EPO          92/908057.0         2/27/92                   Pending

Canada       2105300             2/27/92                   Pending

Japan        92507558            2/27/92                   Pending

PCT          WO9215679           2/28/92                   National Phase

EP           92/908799.7         2/28/92                   Pending

Japan        92508216            2/28/92                   Pending
</Table>

                                      - 7 -
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                                   SCHEDULE D

                        LADNER PATENT LICENSE AGREEMENTS

<Table>
<Caption>
COMPANY                                                    EFFECTIVE DATE
<S>                                                           <C>
Affibody Technology                                           4/27/2001
Affimed                                                       8/10/2003
Affymax                                                       7/25/2001
Alexion Antibody Technologies                                 8/31/1999
Amersham Biosciences (Research Kits)                          6/30/1997
Amersham Biosciences (Affinity Separations)                   10/1/2001
Amgen                                                         2/2/2000
Ambit (formerly Aventa Biosciences)                           6/27/2001
AVIVA Antibody Corp.                                          12/20/2001
Bayer                                                         8/31/1999
BioInvent                                                     1/1/2000
Biosite Diagnostics                                           6/1/1998
Burnham Institute, The                                        7/1/1996
Cambridge Antibody Technology                                 12/31/1997
Corvas International, Inc.                                    6/30/1996
Crucell                                                       3/10/1997
Cubist Pharmaceuticals, Inc.                                  7/20/1998
Domantis                                                      2/16/2002
DuPont-Merck Corporation                                      12/31/1996
DuPont De Nemours and Company                                 10/14/2003
Genencor International                                        12/13/2002
Genentech                                                     9/19/2002
Genzyme Corporation                                           3/29/1996
Glaxo SmithKline                                              6/28/1996
GPC-Biotech                                                   3/17/1999
Idexx Laboratories                                            10/2/2000
IGEN International                                            12/31/1996
Invitrogen                                                    4/28/1997
J&J/Ortho-Clinical Diagnostics                                3/9/1998
Karo Bio                                                      3/31/1999
K&P Labs                                                      8/9/2002
Merck                                                         3/31/1996
Micromet AG                                                   10/30/2000
Millennium Biotherapeutics                                    12/31/1997
MoBiTec                                                       12/31/1998
Morphosys                                                     10/4/1996
New England Biolabs                                           3/1/1997
Novagen                                                       10/1/1996
Phage Screen Inc.                                             1/1/1999
Pharmacia Corporation                                         5/30/1997
Praecis Pharmaceuticals Inc.                                  3/30/1996
Promega Corporation                                           7/10/1998
J&J Pharmaceutical Research and Development                   9/30/1997
Stratagene                                                    3/15/1998
</Table>

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