ANNEX I TERMS AND CONDITIONS FOR PURCHASE OF SHARES

Exhibit 1.2

May 19, 2005

Dyax Corp.

300 Technology Square

Cambridge, Massachusetts 02139

Ladies and Gentlemen:

The undersigned (hereinafter, the “Investor”), hereby confirms and agrees with you as follows:

1.             This Purchase Agreement (the “Agreement”) is made as the date hereof between Dyax Corp., a Delaware corporation (the “Company”), and the Investor.

2.             The Company has authorized the sale and issuance of up to 6,315,000 shares (the “Shares”) of the Company’s common stock, par value $0.01 per share (the “Common Stock”), to certain investors (the “Offering”), as more fully described in that certain Placement Agency Agreement (the “Placement Agency Agreement”) dated the date hereof by and between the Company and Deutsche Bank Securities, Inc. and SG Cowen & Co., LLC (the “Placement Agents”), a copy of which is available to the Investor upon request .  All defined terms used herein and not otherwise defined shall have the same meanings ascribed to such terms in the Placement Agency Agreement.

3.             Subject to execution by the Company and the Placement Agents of the Placement Agency Agreement and delivery of a final prospectus, the Company and the Investor agree that the Investor will purchase from the Company, and the Company will issue and sell to the Investor, the number of shares of Common Stock set forth on the signature page hereto, at the purchase price of $4.00 per share, pursuant to the Terms and Conditions for Purchase of the Shares attached hereto as Annex I and incorporated herein by reference as if fully set forth herein. The Investor acknowledges that the Offering is not being underwritten by the Placement Agents and that there is no minimum offering amount. Shares of Common Stock will be credited to the Investor using customary book-entry procedures.

4.             The Investor represents that, except as set forth below, (a) it has had no position, office or other material relationship within the past three years with the Company or persons known to it to be affiliates of the Company and (b) it is not a, and it has no direct or indirect affiliation or association with any, NASD member as of the date hereof.

5.             The Investor confirms that it had full access to all filings made by the Company with the Securities and Exchange Commission, including the registration statement relating to the Shares, and that it was able to read, review, download and print each such filing.

Please confirm that the foregoing correctly sets forth the agreement between us by signing in the space provided below for that purpose.

DYAX CORP.

ANNEX I

TERMS AND CONDITIONS FOR PURCHASE OF SHARES

1.             Agreement to Sell and Purchase the Shares; Subscription Date.

1.1           Upon the terms and subject to the conditions hereinafter set forth, at the Closing (as defined in Section 2 below), the Company will sell to the Investor, and the Investor will purchase from the Company, the number of shares of Common Stock set forth on the signature page of the Purchase Agreement to which this Annex I is attached, at the purchase price set forth therein.

1.2           The Company may enter into agreements similar to this Agreement with certain other investors (the “Other Investors”) and expects to complete sales of the Shares to them. (The Investor and the Other Investors hereinafter collectively are referred to as the “Investors,” and this Agreement and the agreements executed by the Other Investors are hereinafter collectively referred to as the “Agreements”). The Company may accept or reject any one or more Agreements in its sole discretion.

2.             Delivery of the Shares at Closing. The completion of the purchase and sale of the Shares (the “Closing”) shall take place as provided in Section 2 of the Placement Agency Agreement.

The Company’s obligation to issue and sell the Shares to the Investor shall be subject to the accuracy of the representations and warranties made by the Investor and the fulfillment of those undertakings of the Investor to be fulfilled prior to the Closing.

The Investor’s obligation to purchase the Shares shall be subject to the condition that the Placement Agents shall not have (a) terminated the Placement Agency Agreement pursuant to the terms thereof or (b) determined that the conditions to closing in the Placement Agency Agreement have not been satisfied.

3.             Representations, Warranties and Covenants of the Company. The Company hereby represents and warrants to, and covenants with, the Investor, as follows:

3.1           The Company has filed with the Securities and Exchange Commission (the “Commission”) a “shelf” registration statement on Form S-3 (Registration No. 333-113500), which has become effective, relating to the Common Stock of the Company, under the Securities Act of 1933, as amended (the “Act”), and the rules and regulations (collectively referred to as the “Rules and Regulations”) of the Commission promulgated thereunder.  The registration statement, as amended as of the date of this Agreement, including the exhibits and information (if any) deemed to be part of the registration statement pursuant to Rule 430A or 434(d) under the Act, is hereinafter referred to as the “Registration Statement.”  No stop order suspending the effectiveness of the Registration Statement has been issued and, to the Company’s knowledge, no proceeding for that purpose has been initiated or threatened by the Commission.  The Company, if required by the Rules and Regulations of the Commission, proposes to file the Prospectus (as defined below) with the Commission pursuant to Rule 424(b) of the Rules and Regulations.  The Prospectus, in the form in which it is to be filed with the Commission pursuant to Rule 424(b) of the Rules and Regulations, or, if the Prospectus is not to be filed with the Commission pursuant to Rule 424(b), the Prospectus in the form included as part of the Registration Statement at the time the Registration Statement became effective, is hereinafter referred to as the “Prospectus,” except that if any revised prospectus or prospectus supplement shall be provided to the Placement Agents by the Company for use in connection with the offering and sale of the Shares

which differs from the Prospectus (whether or not such revised prospectus or prospectus supplement is required to be filed by the Company pursuant to Rule 424(b) of the Rules and Regulations), the term “Prospectus” shall refer to such revised prospectus or prospectus supplement, as the case may be, from and after the time it is first provided to the Placement Agents for such use.  Any preliminary prospectus or prospectus subject to completion included in the Registration Statement or filed with the Commission pursuant to Rule 424 under the Act is hereafter called a “Preliminary Prospectus.”  Any reference herein to the Registration Statement, any Preliminary Prospectus or the Prospectus shall be deemed to refer to and include the documents incorporated by reference therein pursuant to Item 12 of Form S-3 which were filed under the Securities Exchange Act of 1934, as amended (the “Exchange Act”) on or before the last to occur of the effective date of the Registration Statement, the date of the Preliminary Prospectus, or the date of the Prospectus, and any reference herein to the terms “amend,” “amendment” or “supplement” with respect to the Registration Statement, any Preliminary Prospectus or the Prospectus shall be deemed to refer to and include (i) the filing of any document under the Exchange Act after the effective date of the Registration Statement, the date of such Preliminary Prospectus or the date of the Prospectus, as the case may be, which is incorporated therein by reference and (ii) any such document so filed.

3.2           As of the date hereof, and at the Closing, the Registration Statement (and any post-effective amendment thereto) and the Prospectus (as amended or as supplemented if the Company shall have filed with the Commission any amendment or supplement to the Registration Statement or the Prospectus) complied and will comply in all material respects with the Act and the Rules and Regulations, and did not and will not contain any untrue statement of a material fact or omit to state any material fact required to be stated therein or necessary to make the statements therein (in the light of the circumstances under which they were made, in the case of the Prospectus) not misleading, each Preliminary Prospectus, as of the date filed with the Commission, did not include any untrue statement of a material fact or omit to state any material fact required to be stated therein or necessary to make the statements therein, in the light of the circumstances under which they were made, not misleading; except that no representation or warranty is made in this Section 3.2 with respect to statements or omissions made in reliance upon and in conformity with written information furnished to the Company by the Placement Agents expressly for inclusion in any Preliminary Prospectus, the Registration Statement, or the Prospectus, or any amendment or supplement thereto, it being understood and agreed upon that the only such information furnished to the Company consists of the following: the statements set forth (i) under the heading “Plan of Distribution,” (ii) on the cover page and (iii) on the back cover in any Preliminary Prospectus and the Prospectus.

3.3           The consolidated financial statements and the related notes thereto included or incorporated by reference in the Registration Statement and the Prospectus comply with the applicable requirements of the Act and the Exchange Act, as applicable, and present fairly, the consolidated financial position of the Company as of the dates indicated and its consolidated results of operations and its consolidated cash flows for the periods specified; such financial statements have been prepared in conformity with generally accepted accounting principles applied on a consistent basis throughout the periods covered thereby (except as otherwise stated therein and subject, in the case of unaudited financial statements, to the absence of footnotes and normal year end adjustments), and the other financial information included or incorporated by reference in the Registration Statement and the Prospectus has been derived from the accounting records of the Company and its Subsidiaries and presents fairly the information shown thereby.

3.4           The Company and its subsidiaries listed in Exhibit 22.1 to the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2004 (the “Subsidiaries”) are each duly organized, validly existing as a corporation and in good standing under the laws of their respective jurisdictions of organization.  The Company and its Subsidiaries are each duly licensed or qualified as a foreign corporation for transaction of business and in good standing under the laws of each other jurisdiction in

which their respective ownership or lease of property or the conduct of their respective businesses requires such license or qualification, and have all corporate power and authority necessary to own or hold their respective properties and to conduct their respective businesses as described in the Registration Statement and the Prospectus, except where the failure to be so qualified or in good standing or have such power or authority would not, individually or in the aggregate, have a material adverse effect or would reasonably be expected to have a material adverse effect on the business, properties, management, consolidated financial position, stockholders’ equity or results of operations of the Company and its Subsidiaries taken as a whole.

3.5           The issued and outstanding shares of capital stock of the Company have been validly issued, are fully paid and nonassessable and, other than as disclosed in or contemplated by the Registration Statement or the Prospectus, are not subject to any preemptive or similar rights.  The Company has an authorized, issued and outstanding capitalization as set forth in the Registration Statement and the Prospectus as of the dates referred to therein (other than the grant of additional options under the Company’s existing stock option plans, or changes in the number of outstanding shares of Common Stock of the Company due to the issuance of shares upon the exercise or conversion of securities exercisable for, or convertible into, shares of Common Stock outstanding on the date hereof, including without limitation issuances of shares under the Company’s employee stock purchase plan) and such authorized capital stock conforms to the description thereof set forth in the Registration Statement and the Prospectus.  The description of the securities of the Company in the Registration Statement and the Prospectus is complete and accurate in all material respects.  Except as disclosed in or contemplated by the Registration Statement or the Prospectus, as of the date referred to therein, the Company did not have outstanding any options to purchase, or any rights or warrants to subscribe for, or any securities or obligations convertible into, or exchangeable for, or any contracts or commitments to issue or sell, any shares of capital stock or other securities.

3.6           The Shares have been duly authorized by the Company and, when issued and delivered and paid for as provided herein, will be validly issued, fully paid and nonassessable and will conform to the descriptions thereof in the Prospectus; and the issuance of the Shares is not subject to any preemptive or similar rights that have not been waived.

3.7           This Agreement constitutes a valid and binding obligation of the Company enforceable against the Company in accordance with its terms, except as enforceability may be limited by applicable bankruptcy, insolvency, reorganization, moratorium or similar laws affecting creditors’ and contracting parties’ rights generally and except as enforceability may be subject to general principles of equity (regardless of whether such enforceability is considered in a proceeding in equity or at law).

4.             Representations, Warranties and Covenants of the Investor.

4.1           The Investor represents and warrants that it has received the Company’s Prospectus.

4.2           The Investor further represents and warrants to, and covenants with, the Company that (i) such Investor has full right, power, authority and capacity to enter into this Agreement and to consummate the transactions contemplated hereby and has taken all necessary action to authorize the execution, delivery and performance of this Agreement, and (ii) this Agreement constitutes a valid and binding obligation of such Investor enforceable against such Investor in accordance with its terms, except as enforceability may be limited by applicable bankruptcy, insolvency, reorganization, moratorium or similar laws affecting creditors’ and contracting parties’ rights generally and except as enforceability may be subject to general principles of equity (regardless of whether such enforceability is considered in a proceeding in equity or at law).

4.3           The Investor represents and warrants to, and covenants with, the Company that: (i) the Investor is knowledgeable, sophisticated and experienced in making, and is qualified to make, decisions with respect to investments in shares representing an investment decision like that involved in the purchase of the Shares; and (ii) the Investor has, in connection with its decision to purchase the number of Shares set forth on Schedule I to the Agreement, relied solely upon the registration statement, the prospectus, and any amendments or supplements thereto and has not relied upon any information provided by Deutsche Bank Securities Inc. and SG Cowen & Co., LLC in their capacity as Placement Agents for the Company.

4.4           The Investor understands that nothing in the prospectus and any supplement thereto, this Agreement or any other materials presented to such Investor in connection with the purchase and sale of the Shares constitutes legal, tax or investment advice. Such Investor has consulted such legal, tax and investment advisors as it, in its sole discretion, has deemed necessary or appropriate in connection with its purchase of the Common Stock.

4.5           From and after obtaining knowledge of the sale of the Shares contemplated hereby, such Investor has not taken, and prior to the public announcement of the transaction such Investor shall not take, any action that has caused or will cause such Investor to have, directly or indirectly, sold or agreed to sell any Common Stock, effected any short sale, whether or not against the box, established any “put equivalent position” (as defined in Rule 16a-1(h) under the Securities Exchange Act of 1934, as amended) with respect to the Common Stock, granted any other right (including, without limitation, any put or call option) with respect to the Common Stock or with respect to any security that includes, relates to or derives any significant part of its value from the Common Stock, whether or not, directly or indirectly, in order to hedge its position in the Shares.

5.             Survival of Representations, Warranties and Agreements. Notwithstanding any investigation made by any party to this Agreement, all covenants, agreements, representations and warranties made by the Company and the Investor herein shall survive the execution of this Agreement, the delivery to such Investor of the Shares being purchased and the payment therefor.

6.             Notices. All notices, requests, consents and other communications hereunder shall be in writing, shall be mailed (A) if within domestic United States by first-class registered or certified airmail, or nationally recognized overnight express courier, postage prepaid, or by facsimile, or (B) if delivered from outside the United States, by International Federal Express or facsimile, and shall be deemed given (i) if delivered by first-class registered or certified mail domestic, three business days after so mailed, (ii) if delivered by a nationally recognized overnight carrier, one business day after so mailed, (iii) if delivered by International Federal Express, two business days after so mailed, (iv) if delivered by facsimile, upon electronic confirmation of receipt and shall be delivered as addressed as follows: (a) if to the Company, then as provided in Section 10 of the Placement Agency Agreement; and (b) if to an Investor, at its address on Schedule I hereto, or at such other address or addresses as may have been furnished to the Company in writing.

7.             Changes. This Agreement may not be modified or amended except pursuant to an instrument in writing signed by the Company and the Investor.

8.             Headings. The headings of the various sections of this Agreement have been inserted for convenience or reference only and shall not be deemed to be part of this Agreement.

9.             Severability. In case any provision contained in this Agreement should be invalid, illegal or unenforceable in any respect, the validity, legality and enforceability of the remaining provisions contained herein shall not in any way be affected or impaired thereby.

10.           Governing Law. This Agreement shall be governed by, and construed in accordance with, the internal laws of the State of New York, without giving effect to the principles of conflicts of law.

11.           Counterparts. This Agreement may be executed in two or more counterparts, each of which shall constitute an original, but all of which, when taken together, shall constitute one instrument, and shall become effective when one or more counterparts have been signed by each party hereto and delivered to the other parties.