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                                                                EXHIBIT 10.5

                                   DYAX CORP.

                       RESTRICTED STOCK PURCHASE AGREEMENT


         This Restricted Stock Purchase Agreement (this "Agreement") is entered
into as of November 30, 1999 (the "Effective Date"), by and between Dyax Corp.,
a Delaware corporation (the "Company"), and Stephen S. Galliker ("Purchaser").

                                    RECITALS

         A. In consideration for the reduction of the stock option award of
75,000 shares to which the Purchaser is entitled pursuant to a letter agreement
between the Company and the Purchaser dated August 25, 1999 to 52,500 shares,
the Company is granting this award of restricted stock.

         B. The Company desires to issue and sell, and Purchaser desires to
purchase, shares of Common Stock of the Company, on the terms and conditions
hereinafter set forth, as "Restricted Stock" issued pursuant to the Company's
1995 Equity Incentive Plan (as amended to date, the "Plan"). Capitalized terms
not otherwise defined herein have the meanings given to them in the Plan.

         Accordingly, it is agreed between the parties as follows:

         1.     Purchaser hereby agrees to purchase from the Company, and
the Company agrees to sell to Purchaser, 22,500 shares (hereinafter sometimes
collectively referred to as the "Stock") of the Company's Common Stock, par
value $0.01 per share, for a purchase price of $2.00 per share, payable at
the closing in cash. The closing hereunder shall be deemed to have occurred
on the Effective Date or at such later date, not later than fifteen days
thereafter, as the Company receives payment of the purchase price.

         2.     For the purposes of this Section 2, the shares of Stock
purchased pursuant to this Agreement shall vest in twenty-four (24)
substantially equal monthly installments (any fractional shares to be
cumulated and to become exercisable at the end of the earliest succeeding
monthly period in which a whole share equivalent is accumulated) beginning on
October 1, 1999, with all shares being fully vested on November 1, 2001. All
of the shares of Stock being purchased by Purchaser pursuant to this
Agreement that are not vested (the "Unvested Shares") shall be subject to the
repurchase right ("Repurchase Right") set forth in this Section 2.

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                  (a) If Purchaser shall cease to employed by the Company, then
the Company may exercise the Repurchase Right, as provided in Section 2(b)
below, at the $2.00 purchase price per share set forth in Section 1 above (the
"Repurchase Price") with respect to all the Unvested Shares as of the date of
such termination. Notwithstanding the foregoing, all shares of Stock shall be
subject to the First Refusal Right set forth in Section 3 hereof.

                  (b) In the event Purchaser shall cease to be employed by the
Company (including a parent or subsidiary of the Company, if any), the Company
shall have the right, at any time within sixty (60) days after the date
Purchaser's employment ceases, to exercise the Repurchase Right, which consists
of the right to purchase from Purchaser (or his personal representative, as the
case may be) all Unvested Shares at the Repurchase Price.

                  (c) If Purchaser is required to transfer his shares of Stock
by operation of law (other than death of Purchaser) or by order or decree of any
court, then the Company shall have the option, exercisable at any time during
the period of sixty (60) days after receiving notice thereof, to purchase all of
the Unvested Shares for the Repurchase Price and upon the terms as set forth in
Section 4. Failure of the Company to elect to purchase Purchaser's Unvested
Shares of Stock under this Section 2 shall not affect the right to purchase the
same shares under Section 3(a) in the event of a proposed sale, assignment,
transfer, exchange, pledge or other disposition by or to any receiver,
petitioner, assignee, transferee or other person obtaining an interest in said
shares.

                  (d) Nothing in this Agreement shall affect in any manner
whatsoever the right or power of the Company, or a parent or subsidiary of the
Company, to terminate Purchaser's provision of services to the Company, for any
reason, with or without cause.

         3.      (a) So long as the First Refusal Right has not terminated as
set forth in Section 3(f) below, Purchaser shall not sell or otherwise transfer
any shares of Stock unless prior to any sale or other transfer thereof,
Purchaser (or his personal representative, as the case may be) shall provide the
Company with written notice, in the manner provided in Section 14 hereof,
describing the number of shares of Stock intended to be sold or transferred, the
price and the general terms of the proposed sale or transfer.

                  (b) The Company shall have the right (the "First Refusal
Right") at any time within sixty (60) days after the notice required by Section
3(a) above to purchase from Purchaser (or his personal representative, as the
case may be) up to but not exceeding the number of shares of the Stock specified
in, and at the price (the "First Refusal Price") and upon the general terms
specified in such notice.

                  (c) If the First Refusal Right is not exercised with respect
to some or all the shares of the Stock specified in the notice required by
Section 3(a) hereof, then for a period of 120 days, Purchaser (or his personal
representative, as the case may be) shall be free to sell, or otherwise
transfer, up to but not exceeding the number of shares of the Stock specified in
the notice required by Section 3(a) hereof, minus the number of shares of the
Stock with respect to which the First Refusal Right was exercised, at a price
and upon general terms no more favorable to purchasers or transferees thereof
than specified in the notice required by Section 3(a) hereof.


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                  (d) In the event that any shares of the Stock which are free
to be sold or otherwise transferred within said 120-day period are not so
transferred, such shares of the Stock shall again be subject to the First
Refusal Right and Purchaser (or his personal representative, as the case may be)
shall comply with all the provisions of this Section 3 prior to selling or
otherwise transferring any such shares of the Stock.

                  (e) Failure to exercise the First Refusal Right with respect
to any shares of the Stock shall not constitute a waiver of the First Refusal
Right with respect to any other shares of the Stock.

                  (f) The First Refusal Right shall continue after the
Purchaser's death or the termination of the Purchaser's association with the
Company for any reason, or no reason, and shall terminate only (1) upon the
closing of an underwritten public offering pursuant to an effective registration
statement under the Securities Act of 1933, as amended that (i) causes an
"Automatic Conversion Event," as such term is defined in the Company's Restated
Certificate of Incorporation in effect on the date hereof, or otherwise (ii)
results in the conversion of all of the Company's outstanding Preferred Stock
into Common Stock by agreement of the requisite number of holders thereof; or
(2) upon the consolidation, merger or sale of all, or substantially all of the
assets of the Company in which securities of the acquiring company (or any
affiliate of the acquiring company) are issued to the Purchaser in exchange for
the Stock, provided that such securities are issued by a publicly held
corporation.

         4.     The Repurchase Right and the First Refusal Right shall be
exercised by written notice signed by an officer of the Company and delivered or
mailed as provided in Section 14 hereof. The Repurchase Price or the First
Refusal Price, as the case may be, shall be payable, at the option of the
Company, in cancellation of all or a portion of any outstanding indebtedness of
Purchaser to the Company or in cash (by check) or both.

         5.     The Company may assign its rights under Sections 2 and 3 hereof.

         6.     If, from time to time during the term of the Repurchase Right or
the First Refusal Right:

                  (a) There is any stock dividend or liquidating dividend of
cash or property, stock split or other change in the character or amount of any
of the outstanding securities of the Company; or

                  (b) There is any consolidation, merger or sale of all or
substantially all, of the assets of the Company unless such consolidation,
merger or sale is with a publicly-owned corporation and the aggregate market
value of the securities or other property the stockholders of the Company
receive is in excess of Twenty Million Dollars ($20,000,000), then, in such
event, any and all new, substituted or additional securities or other property
(other than cash) to which Purchaser is entitled by reason of his ownership of
Stock shall be immediately subject to the Repurchase Right or the First Refusal
Right, as the case may be, and be included in the word "Stock" for all purposes
of the Repurchase Right and the First Refusal Right with the same force and
effect as the shares of Stock subject to the Repurchase Right and the First
Refusal Right under the terms of Section 2 and 3 hereof. While the total
Repurchase Price shall remain the



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same after each such event, the Repurchase Price per share of Stock upon
exercise of the Repurchase Right shall be appropriately adjusted. Stock
acquired as provided in clauses (a) or (b) above shall be deemed to have been
acquired at the time of acquisition of the Stock on which such Stock was
distributed.

         7.      All certificates representing any shares of Stock subject to
the provisions of this Agreement shall have endorsed thereon the following
legends:

                  (a) "Any disposition of any interest in the securities
represented by this certificate is subject to restrictions, and the securities
represented by this certificate are subject to a repurchase right and a first
refusal right contained in a certain agreement between the record holder hereof
and the corporation, a copy of which will be mailed to any holder of this
certificate without charge after receipt by the corporation of a written request
therefor."

                  (b) Any legend required to be placed thereon by federal or
state securities laws.

         8.      Purchaser acknowledges that he is aware that the Stock to be
issued to him by the Company pursuant to this Agreement has not been
registered under the Securities Act of 1933, as amended. Purchaser also
warrants and represents to the Company as follows:

                  (a) Purchaser is purchasing the Stock solely for his own
account for investment and not with a view to or for sale or distribution of the
Stock or any portion thereof and not with any present intention of selling,
offering to sell or otherwise disposing of or distributing the Stock or any
portion thereof. Purchaser also represents that the entire legal and beneficial
interest of the Stock which Purchaser is purchasing is being purchased for, and
will be held for the account of, Purchaser only and neither in whole nor in part
for any other person.

                  (b) Purchaser has heretofore discussed the Company and its
plans, operations and financial condition with its officers and Purchaser has
heretofore received all such information as Purchaser deems necessary and
appropriate to enable Purchaser to evaluate the financial risk inherent in
making an investment in the Stock of the Company and Purchaser further
represents and warrants that Purchaser has received satisfactory and complete
information concerning the business and financial condition of the Company in
response to all inquiries in respect thereof.

                  (c) Purchaser realizes that his purchase of the Stock will be
a highly speculative investment and that Purchaser is able, without impairing
his financial condition, to hold the Stock for an indefinite period of time and
to suffer a complete loss on his investment.

                  (d) The Company has disclosed (or hereby does disclose) to
Purchaser in writing:

                           (i)         the sale of the Stock which he is
                                       purchasing has not been registered under
                                       the Securities Act of 1933, as amended
                                       (the "Act"), and the Stock must be held
                                       indefinitely unless a transfer is
                                       subsequently registered under the Act or
                                       an exemption from such registration is
                                       available;


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                           (ii)        the share certificate representing the
                                       Stock will be stamped with the legends
                                       restricting transfer specified in this
                                       Agreement between the Company and
                                       Purchaser; and

                           (iii)       the Company will make a notation in its
                                       records of the aforementioned
                                       restrictions on transfer and legends.

                  (e) Purchaser understands that the shares of Stock are
restricted securities within the meaning of Rule 144 promulgated under the Act;
that the exemption from registration under Rule 144 will not be available in any
event for at least one year from the date of sale of the Stock to him, and even
then will not be available unless (i) a public trading market then exists for
the Stock of the Company, (ii) adequate current public information concerning
the Company is then available to the public, (iii) he has been the beneficial
owner and he has paid the full purchase price for the Stock at least one year
prior to the sale, and (iv) other terms and conditions of Rule 144 are complied
with; and that any sale of the Stock may be made by only in limited amounts in
accordance with such terms and conditions, as amended from time to time.

                  (f) Without in any way limiting its representations set forth
above, Purchaser further agrees that he shall in no event make any disposition
of all or any portion of the Stock which he is purchasing unless and until:

                           (i)         There is then in effect a Registration
                                       Statement under the Act covering such
                                       proposed disposition and such disposition
                                       is made in accordance with said
                                       Registration Statement: or

                            (ii)       (A) He shall have notified the Company
                                       of the proposed disposition and shall
                                       have furnished the Company with a
                                       detailed statement of circumstances
                                       surrounding the proposed disposition,
                                       (B) he shall have furnished the Company
                                       with an opinion of his own counsel to
                                       the effect that such disposition will
                                       not require registration of such shares
                                       under the Act, and (C) such opinion of
                                       his counsel shall have been concurred in
                                       by counsel for the Company and the
                                       Company shall have advised him of such
                                       concurrence.

          9.       The Company shall not be required (a) to transfer on its
books any shares of Stock of the Company which shall have been sold or
transferred in violation of any of the provisions set forth in this Agreement
or (ii) to treat as owner of such shares or to accord the right to vote as
such owner or to pay dividends to any transferee to whom such shares shall
have been so transferred. In the event of a sale of Stock by Purchaser
pursuant to Section 3(c) hereof, Purchaser shall furnish to the Company proof
that such sale was made in compliance with the provisions of Section 3(c)
hereof as to price and general terms of such sale.

          10.      Subject to the provisions of this Agreement, Purchaser
shall, during the term of this Agreement, exercise all rights and privileges
of a shareholder of the Company with respect to the Stock.

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          11.      Nothing in this Agreement shall affect in any manner
whatsoever the right or power of the Company, or a parent or subsidiary of
the Company, to terminate the Purchaser's provision of services to the
Company, for any reason, with or without cause.

          12.      If Purchaser makes an election under Section 83(b) of the
Code, he will provide a copy thereof to the Company within thirty days of the
filing of such election with the Internal Revenue Service.

          13.      Purchaser shall pay to the Company, or make provision
satisfactory to the Committee for payment of, any taxes required by law to be
withheld in respect of the Restricted Stock no later than the date of the
event creating the tax liability. In the Committee's discretion, such tax
obligations may be paid in whole or in part in shares of Common Stock,
including the Restricted Stock, valued at Fair Market Value on the date of
delivery. The Company and its Affiliates may, to the extent permitted by law,
deduct any such tax obligations from any payment of any kind otherwise due to
Purchaser.

          14.      Any determination by the Committee under, or
interpretation of the terms of, this Agreement or the Plan will be final and
binding on Purchaser.

          15.      Any notice required or permitted hereunder by any party
hereto shall be given in writing and shall be deemed effectively given upon
personal delivery or upon deposit in the United States Post Office, by
registered or certified mail with postage and fees prepaid, addressed to the
other party hereto at such other party's address hereinafter shown below such
other party's signature or at such other address as such other party may have
designated by ten days' advance written notice to such party.

          16.      This Agreement shall inure to the benefit of the
successors and assigns of the Company and be binding upon Purchaser and his
heirs, executors, administrators, successors and assigns.

          17.      This Agreement shall be governed by and interpreted under
the laws of the State of Delaware.

          18.      This Agreement may be executed simultaneously in any
number of counterparts, each of which shall be deemed to be an original, but
all of which together shall constitute one and the same instrument.

          19.      This Agreement constitutes the entire agreement and
understanding between the parties hereto with respect to the subject matter
hereof and supersedes all prior agreements and understanding relating to such
subject matter, including without limitation a Restricted Stock Purchase
Agreement dated as of November 1999 between the parties.

             [The remainder of this page left blank intentionally.]


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         IN WITNESS WHEREOF, the parties hereto have executed this Agreement
under seal as of the day and year first above written.

                                   DYAX CORP.


                                   By: /s/ Henry E. Blair
                                       -----------------------------------
                                       Henry E. Blair
                                       President

                                       Address: One Kendall Square
                                                Building 600, 5th Floor
                                                Cambridge, MA  02139


                                   PURCHASER:


                                   /s/ Stephen S. Galliker
                                   ---------------------------------------
                                   Stephen S. Galliker, individually

                                          Address: 14 Sheep Pasture Way
                                                   East Sandwich, MA 02563