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                                                                 EXHIBIT 10.23



                                                       [Dyax Logo] Biotage, Inc.
                                                            A Dyax Corp. Company


                                      September 27, 2002


Mr. David B. Patteson
1600 Far Hills Ways
Charlottesville, VA  22901

Dear David:

     In recognition of the fact that your contributions to the past and future
growth and success of Biotage, Inc. (the "Company"), a Delaware corporation,
have been and are expected to be substantial, we want to assure the Company of
your continued services for the benefit of the Company, particularly in the face
of a change-in-control of the Company.

     This letter agreement (this "Agreement") therefore sets forth those
benefits which the Company will provide to you in the event your employment
within the Company is terminated after a "Change in Control" (as defined in
Paragraph 2(i)) of the Company under the circumstances described below.

1.   TERM.

     If a Change in Control should occur while you are still an employee of the
Company, then this Agreement shall continue in effect from the date of such
Change in Control for so long as you remain an employee of the Company, but in
no event for more than twenty-four (24) months following such Change in Control.
If your employment is terminated by the Company without Cause (as defined in
Paragraph 3(ii)) prior to a Change in Control, this Agreement shall expire 180
days after the date that your employment is terminated. In addition, this
Agreement may be terminated by the Company at any time upon one year's written
notice to you. The termination or expiration of the term of this Agreement shall
not adversely affect your rights under this Agreement that have accrued prior to
any such termination or expiration.

2.   CHANGE IN CONTROL.

     (i)     For purposes of this Agreement, a Change in Control of the Company
(a "Change in Control") shall be deemed to have occurred only if any of the
following events occur:

             (a) the acquisition of 50% or more of the Common Stock of the
Company (including shares convertible into Common Stock) by any "person" (as
such term is defined in Section 3(a)(9) of the Securities Exchange Act of 1934
(as amended, the "Exchange Act")) other than Dyax Corp. or any wholly-owned
subsidiary thereof or any successor thereto by way of merger or other operation
of law;

             (b) a merger or similar combination after which 50% or more of the
voting stock of the Company or any other surviving corporation that is the
successor to the Company is not held by Dyax or the other persons having

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Mr. David B. Patteson
September 27, 2002
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             (c)    beneficial ownership (within the meaning of Rule 13d-3 under
the Exchange Act) of the Company immediately prior to such merger or combination
(excluding for this purpose any shares held by a person other than Dyax who
holds beneficial ownership of more than 50% of the voting stock of the surviving
corporation immediately after such merger or combination); or

             (d)    the sale, lease, or exchange of all or substantially all of
the Company's property or assets, or the dissolution or liquidation of the
Company;

     provided, however, that a spin-off transaction in which ownership of the
     stock of the Company is merely transferred pro rata to the stockholders of
     Dyax or its successor shall not be deemed to be a Change in Control of the
     Company.

3.   TERMINATION FOLLOWING CHANGE IN CONTROL.

     If a Change in Control shall have occurred while you are still an employee
of the Company, you shall be entitled to the payments and benefits provided in
Paragraph 4 hereof upon the subsequent termination of your employment within
twenty-four (24) months of such Change in Control, by you or by the Company
unless such termination is (a) because of your death or "Disability", (b) by the
Company for "Cause" (as defined below), or (c) by you other than for "Good
Reason" (as defined below), in any of which events you shall not be entitled to
receive benefits under this Agreement.

     (i)     "DISABILITY". If, as a result of your incapacity due to
physical or mental illness, you shall have been deemed "disabled" by the
institution appointed by the Company to administer the Company's Long-Term
Disability Plan (or successor plan) because you shall have been absent from your
duties with the Company on a full-time basis and shall not have returned to
full-time performance of your duties within thirty days after written notice is
given you, the Company may terminate your employment for Disability.

     (ii)    "CAUSE".  For the purpose of this Agreement, the Company shall have
"Cause" to terminate your employment upon

             (a)    The willful and continued failure by you substantially to
perform your duties with the Company (other than any such failure resulting from
your incapacity due to physical or mental illness or any failure resulting from
your terminating your employment with the Company for "Good Reason" (as defined
below)) after a written demand for substantial performance is delivered to you
by the Company which specifically identifies the manner in which the Company
believes that you have not substantially performed your duties;

             (b)    Willful gross misconduct or dishonesty; or

             (c)    Conviction of a felony or a crime involving moral turpitude.

     (iii)   "GOOD REASON".  You may terminate your employment for Good Reason.
For purpose of this Agreement, "Good Reason" shall mean:

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Mr. David B. Patteson
September 27, 2002
Page 3
             (a)    The material diminution of your duties with the Company from
that immediately prior to the Change in Control;

             (b)    A reduction by the Company in your base salary in effect
immediately prior to the Change in Control;

             (c)    Any requirement by the Company that (i) the location at
which you perform your principal duties for the Company be changed to a new
location that is more than 50 miles from the location at which you perform your
principal duties for the Company immediately prior to the Change in Control.

     (iv)    NOTICE OF TERMINATION. Any termination by the Company pursuant to
subparagraphs (i) or (ii) above or by you pursuant to subparagraph (iii) above
shall be communicated by written Notice of Termination to the other party
hereto. For purposes of this Agreement, a "Notice of Termination" shall mean a
notice which shall indicate the specific termination provision in this Agreement
relied upon and shall set forth in reasonable detail the facts and circumstances
claimed to provide a basis of your termination under the provision so indicated.

     (v)     DATE OF TERMINATION.  "Date of Termination" shall mean:

             (a)    If this Agreement is terminated for Disability, thirty days
after Notice of Termination is given (provided that you shall not have returned
to the performance of your duties on a full-time basis during such thirty-day
period),

             (b)    If your employment is terminated pursuant to subparagraph
(iii) above, the date specified in the Notice of Termination; and

             (c)    If your employment is terminated for any other reason, the
date on which a Notice of Termination is given (or, if a Notice of Termination
is not given, the date of such termination).

     (vi)    Termination in Anticipation of a Change in Control. If your
employment is terminated by the Company without Cause within 180 days prior to a
Change of Control and such termination (i) was at the request of a third party
who had indicated an intention or had taken steps reasonably calculated to
effect a Change of Control and who subsequently effectuates a Change of Control
(a "Third Party") or (ii) otherwise occurred as a condition to, or in
anticipation of, a Change of Control which actually accors, then for all
purposes of this Agreement, the date of a Change of Control for purposes of this
Agreement shall mean the date immediately prior to the date of such termination
of your employment and shall entitle you to the benefits provided under Section
4 of this Agreement as though it were a termination without Cause after a Change
in Control.

4.   COMPENSATION DURING DISABILITY OR UPON TERMINATION AFTER A CHANGE IN
CONTROL.

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Mr. David B. Patteson
September 27, 2002
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     (i)     If, after a Change in Control, you shall fail to perform your
duties hereunder as a result of incapacity due to Disability, you shall continue
to receive your full base salary at the rate then in effect until your
employment is terminated (and, if the Company maintains a Long Term Disability
Plan, you shall be eligible for coverage thereunder in accordance with the terms
thereof and subject to the satisfaction of all applicable conditions, including
without limitation the timely filing of a notice of claim);

     (ii)    If, after a Change in Control, your employment shall be terminated
for Cause, the Company shall pay you for your full base salary through the Date
of Termination at the rate in effect at the time Notice of Termination is given
and the Company shall have no further obligations to you under this Agreement.

     (iii)   If, after a Change in Control, the Company shall terminate your
employment, other than pursuant to Paragraph 3(i) or (ii) hereof or by reason of
death, or you shall terminate your employment for Good Reason:

             (a)    The Company shall pay you as severance pay (and without
regard to the provisions of any benefit plan) twenty-four (24) months of salary
continuation at the base rate in effect at the time of termination (together
with the payments provided in paragraphs (b), (c) and (d) below, the "Severance
Payments");

             (b)    For a twenty-four (24) month period after such termination,
the Company shall arrange to provide you with life, dental, accident and group
health insurance benefits substantially similar to those that you were receiving
immediately prior to such termination to the extent that the Company's plans
then permit the Company to provide you with such benefits. Notwithstanding the
foregoing, the Company shall not provide any such benefits to you to the extent
that an equivalent benefit is received by you from another employer during such
period, and you shall report any such benefit actually received by you to the
Company;

             (c)    The exercisability of all outstanding stock options and
restricted stock awards with respect to common stock of Dyax Corp. then held by
you shall accelerate in full; and

             (d)    You shall be entitled to full executive outplacement
assistance with an agency selected by the Company.

     (iv)    You shall not be required to mitigate the amount of any payment
provided for in this Paragraph 4 by seeking other employment or otherwise, nor
shall the amount of any payment provided for in this Paragraph 4 be reduced by
any compensation earned by you as the result of employment by another employer
after the Date of Termination, or otherwise, except to the extent provided in
Section 4(iii)(b) above.

5.   LIMIT ON SEVERANCE PAYMENTS.

     In the event that any payment or benefit received or to be received by you
in connection with a Change in Control or the termination of your employment
(whether pursuant to the terms of this Agreement or any other plan, arrangement
or agreement with the Company, any person

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Mr. David B. Patteson
September 27, 2002
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whose actions result in a Change in Control or any person affiliated with the
Company or such person) (collectively with the Severance Payments, "Total
Payments") would not be deductible (in whole or part) as a result of section
280G of the Internal Revenue Code of 1986, as amended (the "Code) by the
Company, an affiliate or other person making such payment or providing such
benefit, the Severance Payments shall be reduced until no portion of the Total
Payments is not deductible, or the Severance Payments are reduced to zero. For
purposes of this limitation (a) no portion of the Total Payments the receipt or
enjoyment of which you shall have effectively waived in writing prior to the
date of payment of the Severance Payments shall be taken into account, (b) no
portion of the Total Payments shall be taken into account which in the opinion
of tax counsel selected by the Company's independent auditors serving as such
immediately prior to the Change in Control does not constitute a "parachute
payment" within the meaning of section 280G(b)(2) of the Code, (c) the Severance
Payments shall be reduced only to the extent necessary so that the Total
Payments (other than those referred to in clauses (a) or (b)) in their entirety
constitute reasonable compensation for services actually rendered within the
meaning of section 280G(b)(4) of the Code or are otherwise not subject to
disallowance as deductions, in the opinion of the tax counsel referred to in
clause (b); and (d) the value of any non-cash benefit or any deferred payment or
benefit included in the Total Payments shall be determined by the Company's
independent auditors based on Sections 280G and 4999 of the Code and on proposed
final regulations for applying those Code Section, or on substantial authority
within the meaning of Section 6662 of the Code.

6.   NOTICE.

     For the purpose of this Agreement, notices and all other communications
provided for in this Agreement shall be in writing and shall be delivered to
each party at each party's respective address set forth on the first page of
this Agreement, and shall be deemed effectively given or delivered: (i) upon
personal delivery to the party to be notified, (ii) three (3) days after having
been sent by registered or certified mail, return receipt requested, postage
prepaid, or (iii) one (1) business day after deposit with a nationally
recognized overnight courier, specifying next day delivery, with written
verification of receipt; provided that all notices to the Company should be
directed to the attention of the Chairman of the Board of the Company, with a
copy to the Chief Financial Officer of Dyax Corp., in each case at 300
Technology Square, Cambridge, MA 02139.

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Mr. David B. Patteson
September 27, 2002
Page 6
7.   ENTIRE AGREEMENT.

     This Agreement represents the entire agreement of the parties with respect
to the subject matter hereof and supersedes any other agreement between the
parties with respect to such subject matter.

8.   COUNTERPARTS.

     This Agreement may be executed in one or more counterparts, each of which
shall be deemed to be an original but all of which together will constitute one
in the same instrument.

9.   MISCELLANEOUS.

     (i)     No provision of this Agreement may be modified, waived, or
discharged unless such waiver, modification, or discharge is agreed to in
writing signed by you and such officer as may be specifically designated by the
Board of Directors of the Company.

     (ii)    No waiver by either party hereto at any time of any breach by the
other party hereto of, or compliance with, any condition or provision of this
Agreement to be performed by such other party shall be deemed a waiver of
similar or dissimilar provisions or conditions at the same or at any time prior
to subsequent time.

     (iii)   The validity, interpretation, construction and performance of this
Agreement shall be governed by the laws of the Commonwealth of Virginia.

     (iv)    The invalidity or unenforceability of any provision of this
Agreement shall not affect the validity or enforceability of any other provision
of this Agreement, which shall remain in full force and effect.

     (v)     The Company may withhold from any amounts payable under this
Agreement such federal, state, local or foreign taxes as shall be required to be
withheld pursuant to any applicable law or regulation.

     If this Agreement correctly sets forth our agreement on the subject matter
hereof, kindly sign and return to the Company the enclosed copy of this
Agreement which will then constitute our agreement on this subject.

                               Sincerely,


                               /s/ Henry E. Blair
                               -------------------------------------------------
                               Name:  Henry E. Blair
                               Title: Chief Executive Officer and President

I acknowledge receipt and agree with the foregoing terms and conditions.


 /s/ David B. Patteson
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Mr. David B. Patteson
September 27, 2002
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Name: David B. Patterson