Document:

HIGHTOUCH TECHNOLOGIES, INC.
                          ----------------------------

                             1999 STOCK OPTION PLAN
                             ----------------------

         1.       Purpose.

         The purpose of this plan (the "Plan") is to secure for HighTouch
Technologies, Inc. (the "Company") and its shareholders the benefits arising
from capital stock ownership by employees, officers and directors of, and
consultants or advisors to, the Company and its parent and subsidiary
corporations who are expected to contribute to the Company's future growth and
success. Except where the context otherwise requires, the term "Company" shall
include the parent and all present and future subsidiaries of the Company as
defined in Sections 424(e) and 424(f) of the Internal Revenue Code of 1986, as
amended or replaced from time to time (the "Code"). Those provisions of the Plan
which make express reference to Section 422 shall apply only to Incentive Stock
Options (as that term is defined in the Plan).

         2.       Type of Options and Administration.

                  (a) Types of Options. Options granted pursuant to the Plan
shall be authorized by action of the Board of Directors of the Company (or a
Committee designated by the Board of Directors) and may be either incentive
stock options ("Incentive Stock Options") meeting the requirements of Section
422 of the Code or non-statutory options which are not intended to meet the
requirements of Section 422 of the Code.

                  (b) Administration. The Plan will be administered by the
Board of Directors of the Company, whose construction and interpretation of the
terms and provisions of the Plan shall be final and conclusive. The Board of
Directors may in its sole discretion grant options to purchase shares of the
Company's Common Stock ("Common Stock") and issue shares upon exercise of such
options as provided in the Plan. The Board shall have authority, subject to the
express provisions of the Plan, to construe the respective option agreements and
the Plan, to prescribe, amend and rescind rules and regulations relating to the
Plan, to determine the terms and provisions of the respective option agreements,
which need not be identical, and to make all other determinations which are in
the judgment of the Board of Directors, necessary or desirable for the
administration of the Plan. The Board of Directors may correct any defect,
supply any omission or reconcile any inconsistency in the Plan or in any option
agreement in the manner and to the extent it shall deem expedient to carry the
Plan into effect and it shall be the sole and final judge of such expediency. No
director or person acting pursuant to authority delegated by the Board of
Directors shall be liable for any action or determination under the Plan made in
good faith. The Board of Directors may, to the full extent permitted by or
consistent with applicable laws or regulations (including, without limitation,
applicable state law, Section 162(m) of the Code or any regulations thereunder,
or any successor section of the Code or regulations thereunder ("Section
162(m)"), and Rule 16b-3 promulgated under the Securities Exchange Act of 1934
(the "Exchange Act"), or any successor rule ("Rule 16b-3)"), delegate any or all
of its powers under the Plan to a committee (the "Committee") appointed by the
Board of Directors, and if the Committee is so appointed all references to the
Board of Directors in the Plan shall mean and relate to such Committee. Unless
all members of the Board of Directors are "outside

                                       1
<PAGE>

directors" within the meaning of Section 162(m) of the Code, as such term is
interpreted from time to time, then if the limitations on the tax deductibility
of certain compensation provided by Section 162(m) are applicable to Options
granted under the Plan, the Board shall appoint such Committee of two or more
directors, all of whom are outside directors, and shall delegate to such
Committee all of its powers under the Plan; provided, that any failure of any
director or Committee member to satisfy the definition of outside director shall
not invalidate any action taken by the Board or Committee with respect to any
participant in the Plan, whether or not such person is a "covered employee"
within the meaning of Section 162(m) of the Code, as such term is interpreted
from time to time.

                  (c) Applicability of Rule 16b-3 . Those provisions of the Plan
which make express reference to Rule 16b-3 shall apply only to such persons as
are required to file reports under Section 16(a) of the Exchange Act (a
"Reporting Person").

         3.       Eligibility.

                  (a) General. Options may be granted to persons who are, at
the time of grant, employees, officers or directors of, or consultants or
advisors to, the Company; provided, that the class of employees to whom
Incentive Stock Options may be granted shall be limited to all employees of the
Company. A person who has been granted an option may, if he or she is otherwise
eligible, be granted additional options if the Board of Directors shall so
determine.

                  (b) Grant of Options to Directors and Officers. From and
after the registration of the Common Stock of the Company under the Exchange
Act, the selection of a director or an officer (as the terms "director" and
"officer" are defined for purposes of Rule 16b-3) as a recipient of an option,
the timing of the option grant, the exercise price of the option and the number
of shares subject to the option shall be determined either (i) by the Board of
Directors, or (ii) by two or more directors having full authority to act in the
matter, each of whom shall be a "non-employee director." For the purposes of the
Plan, a director shall be deemed to be a "non-employee director" only if such
person qualifies as a "non-employee director" within the meaning of Rule 16b-3,
as such term is interpreted from time to time.

         4.       Stock Subject to Plan.

         Subject to adjustment as provided in Section 15 below, the maximum
number of shares of Common Stock of the Company which may be issued and sold
under the Plan is Three Hundred Forty Five Thousand Three Hundred Forty Five
(345,345). If an option granted under the Plan shall expire or terminate for any
reason without having been exercised in full, the unpurchased shares subject to
such option shall again be available for subsequent option grants under the
Plan. If shares issued upon exercise of an option under the Plan are tendered to
the Company in payment of the exercise price of an option granted under the
Plan, such tendered shares shall again be available for subsequent option grants
under the Plan; provided, that in no event shall such shares be made available
for issuance to Reporting Persons or pursuant to exercise of Incentive Stock
Options.

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<PAGE>

         5.       Forms of Option Agreements.

         As a condition to the grant of an option under the Plan, each recipient
of an option shall execute an option agreement in such form not inconsistent
with the Plan as may be approved by the Board of Directors. Such option
agreements may differ among recipients.

         6.       Purchase Price.

                  (a) General. The purchase price per share of stock
deliverable upon the exercise of an option shall be determined by the Board of
Directors, provided, however, that in the case of an Incentive Stock Option, or
an option intended to qualify as performance-based compensation within the
meaning of Section 162(m), the exercise price shall not be less than 100% of the
fair market value of such stock, as determined by the Board of Directors, at the
time of grant of such option, or less than 110% of such fair market value in the
case of options described in Section 11(b).

                  (b) Payment of Purchase Price. Options granted under the Plan
may provide for the payment of the exercise price by delivery of cash or a check
to the order of the Company in an amount equal to the exercise price of such
options, or, to the extent provided in the applicable option agreement, (i) by
delivery to the Company of shares of Common Stock of the Company owned by the
optionee having a fair market value equal in amount to the exercise price of the
options being exercised (including shares which the optionee would be entitled
to receive upon exercise of the option), (ii) by any other means (including,
without limitation, by delivery of a promissory note of the optionee payable on
such terms as are specified by the Board of Directors) which the Board of
Directors determines are consistent with the purpose of the Plan and with
applicable laws and regulations (including, without limitation, the provisions
of Rule 16b-3 and Regulation T promulgated by the Federal Reserve Board and
Section 162(m) to the extent an option is intended to qualify as performance
based compensation within the meaning of that Section) or (iii) by any
combination of such methods of payment. The fair market value of any shares of
the Company's Common Stock or other non-cash consideration which may be
delivered upon exercise of an option shall be determined by the Board of
Directors.

         7.       Option Period.

         Each option and all rights thereunder shall expire on such date as
shall be set forth in the applicable option agreement, except that, in the case
of an Incentive Stock Option, such date shall not be later than ten years after
the date on which the option is granted (except as provided in Section 11(b))
and, in all cases, options shall be subject to earlier termination as provided
in the Plan.

         8.       Exercise of Options.

         Each option granted under the Plan shall be exercisable either in full
or in installments at such time or times and during such period as shall be set
forth in the agreement evidencing such option, subject to the provisions of the
Plan.

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<PAGE>

         9.       Nontransferability of Options.

         Incentive Stock Options, and all options granted to Reporting Persons,
shall not be assignable or transferable by the person to whom they are granted,
either voluntarily or by operation of law, except by will or the laws of descent
and distribution, and, during the life of the optionee, shall be exercisable
only by the optionee; provided, however, that non-statutory options may be
transferred pursuant to a qualified domestic relations order (as defined in Rule
16b-3).

         10.      Effect of Termination of Employment or Other Relationship.

         Except as provided in Section 11(d) with respect to Incentive Stock
Options, and subject to the provisions of the Plan, the Board of Directors shall
determine the period of time during which an optionee may exercise an option
following (i) the termination of the optionee's employment or other relationship
with the Company or (ii) the death or disability of the optionee. Such periods
shall be set forth in the agreement evidencing such option.

         11.      Incentive Stock Options.

         Options granted under the Plan which are intended to be Incentive Stock
Options shall be subject to the following additional terms and conditions:

                  (a) Express Designation. All Incentive Stock Options granted
under the Plan shall, at the time of grant, be specifically designated as such
in the option agreement covering such Incentive Stock Options.

                  (b) 10% Shareholder. If any employee to whom an Incentive
Stock Option is to be granted under the Plan is, at the time of the grant of
such option, the owner of stock possessing more than 10% of the total combined
voting power of all classes of stock of the Company (after taking into account
the attribution of stock ownership rules of Section 424(d) of the Code), then
the following special provisions shall be applicable to the Incentive Stock
Option granted to such individual:

                  (i) The purchase price per share of the Common Stock subject
         to such Incentive Stock Option shall not be less than 110% of the fair
         market value of one share of Common Stock at the time of grant; and

                  (ii) The option exercise period shall not exceed ten years
         from the date of grant.

                  (c) Dollar Limitations. For so long as the Code shall so
provide, options granted to any employee under the Plan (and any other incentive
stock option plans of the Company) which are intended to constitute Incentive
Stock Options shall not constitute Incentive Stock Options to the extent that
such options, in the aggregate, become exercisable for the first time in any one
calendar year for shares of Common Stock with an aggregate fair market value
(determined as of the respective date or dates of grant) of more than $100,000.

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<PAGE>

                  (d) Termination of Employment, Death or Disability. No
Incentive Stock Option may be exercised unless, at the time of such exercise,
the optionee is, and has been continuously since the date of grant of his or her
option, employed by the Company, except that:

                  (i) an Incentive Stock Option may be exercised within the
         period of three months after the date the optionee ceases to be an
         employee of the Company (or within such lesser period as may be
         specified in the applicable option agreement), provided, that the
         agreement with respect to such option may designate a longer exercise
         period and that the exercise after such three-month period shall be
         treated as the exercise of a non-statutory option under the Plan;

                  (ii) if the optionee dies while in the employ of the Company,
         or within three months after the optionee ceases to be such an
         employee, the Incentive Stock Option may be exercised by the person to
         whom it is transferred by will or the laws of descent and distribution
         within the period of one year after the date of death (or within such
         lesser period as may be specified in the applicable option agreement);
         and

                  (iii) if the optionee becomes disabled (within the meaning of
         Section 22(e)(3) of the Code or any successor provision thereto) while
         in the employ of the Company, the Incentive Stock Option may be
         exercised within the period of one year after the date the optionee
         ceases to be such an employee because of such disability (or within
         such lesser period as may be specified in the applicable option
         agreement).

For all purposes of the Plan and any option granted hereunder, "employment"
shall be defined in accordance with the provisions of Section 1.421-7(h) of the
Income Tax Regulations (or any successor regulations). Notwithstanding the
foregoing provisions, no Incentive Stock Option may be exercised after its
expiration date.

         12.      Additional Provisions.

         (a) Additional Option Provisions. The Board of Directors may, in its
sole discretion, include additional provisions in option agreements covering
options granted under the Plan, including without limitation restrictions on
transfer, repurchase rights, commitments to pay cash bonuses, to make, arrange
for or guaranty loans or to transfer other property to optionees upon exercise
of options, or such other provisions as shall be determined by the Board of
Directors; provided, that such additional provisions shall not be inconsistent
with any other term or condition of the Plan and such additional provisions
shall not cause any Incentive Stock Option granted under the Plan to fail to
qualify as an Incentive Stock Option within the meaning of Section 422 of the
Code.

          (b) Acceleration, Extension, Etc. The Board of Directors may, in its
sole discretion, (i) accelerate the date or dates on which all or any particular
option or options granted under the Plan may be exercised or (ii) extend the
dates during which all, or any particular option or options granted under the
Plan may be exercised; provided, however, that no such extension shall be
permitted if it would cause the Plan to fail to comply with Section 422 of the
Code or

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<PAGE>

with Rule 16b-3 or cause any option granted under the Plan to fail to
qualify as performance-based compensation within the meaning of Section 162(m).

         13.      General Restrictions.

                  (a) Investment Representation . The Company may require any
person to whom an option is granted, as a condition of exercising such option,
to give written assurances in substance and form satisfactory to the Company to
the effect that such person is acquiring the Common Stock subject to the option
for his or her own account for investment and not with any present intention of
selling or otherwise distributing the same, and to such other effects as the
Company deems necessary or appropriate in order to comply with federal and
applicable state securities laws, or with covenants or representations made by
the Company in connection with any public offering of its Common Stock.

                  (b) Compliance with Securities Laws . Each option shall be
subject to the requirement that if, at any time, counsel to the Company shall
determine that the listing, registration or qualification of the shares subject
to such option upon any securities exchange or under any state or federal law,
or the consent or approval of any governmental or regulatory body, or that the
disclosure of non-public information or the satisfaction of any other condition
is necessary as a condition of, or in connection with, the issuance or purchase
of shares thereunder, such option may not be exercised, in whole or in part,
unless such listing, registration, qualification, consent or approval, or
satisfaction of such condition shall have been effected or obtained on
conditions acceptable to the Board of Directors. Nothing herein shall be deemed
to require the Company to apply for or to obtain such listing, registration or
qualification, or to satisfy such condition.

         14.      Rights as a Shareholder.

                  The holder of an option shall have no rights as a shareholder
with respect to any shares covered by the option (including, without limitation,
any rights to receive dividends or non-cash distributions with respect to such
shares) until the date of issue of a stock certificate to him or her for such
shares. No adjustment shall be made for dividends or other rights for which the
record date is prior to the date such stock certificate is issued.

         15.      Adjustment Provisions for Recapitalizations and Related
                  Transactions.

                  (a) General. If, through or as a result of any merger,
consolidation, sale of all or substantially all of the assets of the Company,
reorganization, recapitalization, reclassification, stock dividend, stock split,
reverse stock split or other similar transaction, (i) the outstanding shares of
Common Stock are increased, decreased or exchanged for a different number or
kind of shares or other securities of the Company, or (ii) additional shares or
new or different shares or other securities of the Company or other non-cash
assets are distributed with respect to such shares of Common Stock or other
securities, an appropriate and proportionate adjustment may be made in (x) the
maximum number and kind of shares reserved for issuance under the Plan, (y) the
number and kind of shares or other securities subject to any then outstanding
options under the Plan, and (z) the price for each share subject to any then
outstanding options under the

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Plan, without changing the aggregate purchase price as to which such options
remain exercisable. Notwithstanding the foregoing, no adjustment shall be made
pursuant to this Section 15 if such adjustment would cause the Plan to fail to
comply with Section 422 of the Code or with Rule 16b-3.

                  (b) Board Authority to Make Adjustments. Any adjustments under
this Section 15 will be made by the Board of Directors, whose determination as
to what adjustments, if any, will be made and the extent thereof will be final,
binding and conclusive. No fractional shares will be issued under the Plan on
account of any such adjustments.

         16.      Merger, Consolidation, Asset Sale, Liquidation, etc.

                  (a) General. In the event of a consolidation or merger or
sale of all or substantially all of the assets of the Company in which
outstanding shares of Common Stock are exchanged for securities, cash or other
property of any other corporation or business entity or in the event of a
liquidation of the Company, the Board of Directors of the Company, or the board
of directors of any corporation assuming the obligations of the Company, may, in
its discretion, take any one or more of the following actions, as to outstanding
options: (i) provide that such options shall be assumed, or equivalent options
shall be substituted, by the acquiring or succeeding corporation (or an
affiliate thereof), provided that any such options substituted for Incentive
Stock Options shall meet the requirements of Section 424(a) of the Code, (ii)
upon written notice to the optionees, provide that all unexercised options will
terminate immediately prior to the consummation of such transaction unless
exercised by the optionee within a specified period following the date of such
notice, (iii) in the event of a merger under the terms of which holders of the
Common Stock of the Company will receive upon consummation thereof a cash
payment for each share surrendered in the merger (the "Merger Price"), make or
provide for a cash payment to the optionees equal to the difference between (A)
the Merger Price times the number of shares of Common Stock subject to such
outstanding options (to the extent then exercisable at prices not in excess of
the Merger Price) and (B) the aggregate exercise price of all such outstanding
options in exchange for the termination of such options, and (iv) provide that
all or any outstanding options shall become exercisable in full immediately
prior to such event, provided, however, the Board of Directors may not take any
action which would cause any option granted under the plan to fail to qualify as
performance-based compensation within the meaning of Section 162(m).

                  (b) Substitute Options. The Company may grant options under
the Plan in substitution for options held by employees of another corporation
who become employees of the Company, or a subsidiary of the Company, as the
result of a merger or consolidation of the employing corporation with the
Company or a subsidiary of the Company, or as a result of the acquisition by the
Company, or one of its subsidiaries, of property or stock of the employing
corporation. The Company may direct that substitute options be granted on such
terms and conditions as the Board of Directors considers appropriate in the
circumstances.

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<PAGE>

         17.      No Special Employment Rights.

         Nothing contained in the Plan or in any option shall confer upon any
optionee any right with respect to the continuation of his or her employment by
the Company or interfere in any way with the right of the Company at any time to
terminate such employment or to increase or decrease the compensation of the
optionee.

         18.      Other Employee Benefits.

         Except as to plans which by their terms include such amounts as
compensation, the amount of any compensation deemed to be received by an
employee as a result of the exercise of an option or the sale of shares received
upon such exercise will not constitute compensation with respect to which any
other employee benefits of such employee are determined, including, without
limitation, benefits under any bonus, pension, profit-sharing, life insurance or
salary continuation plan, except as otherwise specifically determined by the
Board of Directors.

         19.      Amendment of the Plan.

         The Board of Directors may at any time, and from time to time, modify
or amend the Plan in any respect, except that if at any time the approval of the
shareholders of the Company is required under Section 422 of the Code or any
successor provision with respect to Incentive Stock Options, or under Rule
16b-3, or is required to ensure that any compensation resulting from any option
under the Plan is deductible by the Company for federal income tax purposes
under Section 162(m) the Board of Directors may not effect such modification or
amendment without such approval.

         The termination or any modification or amendment of the Plan shall not,
without the consent of an optionee, affect his or her rights under an option
previously granted to him or her. With the consent of the optionee affected, the
Board of Directors may amend outstanding option agreements in a manner not
inconsistent with the Plan. The Board of Directors shall have the right to amend
or modify (i) the terms and provisions of the Plan and of any outstanding
Incentive Stock Options granted under the Plan to the extent necessary to
qualify any or all such options for such favorable federal income tax treatment
(including deferral of taxation upon exercise) as may be afforded incentive
stock options under Section 422 of the Code and (ii) the terms and provisions of
the Plan and of any outstanding option to the extent necessary to ensure the
qualification of the Plan under Rule 16b-3 or to ensure that any compensation
resulting from any option under the Plan is deductible by the Company for
federal income tax purposes under Section 162(m).

         20.      Withholding.

                  (a) The Company shall have the right to deduct from payments
of any kind otherwise due to the optionee any federal, state or local taxes of
any kind required by law to be withheld with respect to any shares issued upon
exercise of options under the Plan. Subject to the prior approval of the
Company, which may be withheld by the Company in its sole discretion, the
optionee may elect to satisfy such obligations, in whole or in part, (i) by
causing

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<PAGE>

the Company to withhold shares of Common Stock otherwise issuable
pursuant to the exercise of an option or (ii) by delivering to the Company
shares of Common Stock already owned by the optionee. The shares so delivered or
withheld shall have a fair market value equal to such withholding obligation.
The fair market value of the shares used to satisfy such withholding obligation
shall be determined by the Company as of the date that the amount of tax to be
withheld is to be determined. An optionee who has made an election pursuant to
this Section 20(a) may only satisfy his or her withholding obligation with
shares of Common Stock which are not subject to any repurchase, forfeiture,
unfulfilled vesting or other similar requirements.

          (b) Notwithstanding the foregoing, in the case of a Reporting Person,
no election to use shares for the payment of withholding taxes shall be
effective unless made in compliance with any applicable requirements of Rule
16b-3.

         21.      Cancellation and New Grant of Options, Etc.

         The Board of Directors shall have the authority to effect, at any time
and from time to time, with the consent of the affected optionees, (i) the
cancellation of any or all outstanding options under the Plan and the grant in
substitution therefor of new options under the Plan covering the same or
different numbers of shares of Common Stock and having an option exercise price
per share which may be lower or higher than the exercise price per share of the
canceled options or (ii) the amendment of the terms of any and all outstanding
options under the Plan to provide an option exercise price per share which is
higher or lower than the then-current exercise price per share of such
outstanding options.

         22.      Effective Date and Duration of the Plan.

                  (a) Effective Date. The Plan shall become effective when
adopted by the Board of Directors, but no Incentive Stock Option granted under
the Plan or nonstatutory option granted under the Plan to an officer shall
become exercisable unless and until the Plan shall have been approved by the
Company's shareholders. If such shareholder approval is not obtained within
twelve months after the date of the Board's adoption of the Plan, no options
previously granted under the Plan shall be deemed to be Incentive Stock Options,
options previously granted to officers shall not vest and shall terminate and no
Incentive Stock Options shall be granted thereafter nor shall any options be
granted thereafter to officers. Amendments to the Plan not requiring shareholder
approval shall become effective when adopted by the Board of Directors;
amendments requiring shareholder approval (as provided in Section 19) shall
become effective when adopted by the Board of Directors, but no Incentive Stock
Option granted after the date of such amendment and no nonstatutory option
granted to any officer after the date of such amendment shall become exercisable
(to the extent that (i) such amendment to the Plan was required to enable the
Company to grant such Incentive Stock Option to a particular optionee or (ii)
shareholder approval of such amendment to the Plan was required to ensure the
deductibility by the Company of any compensation resulting from any option under
Section 162(m)) unless and until such amendment shall have been approved by the
Company's shareholders. If such shareholder approval is not obtained within
twelve months of the Board's adoption of such amendment, (i) any Incentive Stock
Options granted on or after the date of such amendment shall terminate to the
extent that such amendment to the Plan was required to enable the Company to

                                       9
<PAGE>

grant such option to a particular optionee and (ii) any option granted to any
officer on or after the date of such amendment shall terminate to the extent
that shareholder approval of such amendment was required to ensure the
deductibility by the Company of any compensation resulting from any option under
Section 162(m). Subject to this limitation, options may be granted under the
Plan at any time after the effective date and before the date fixed for
termination of the Plan.

                  (b) Termination. Unless sooner terminated in accordance with
Section 16, the Plan shall terminate, with respect to Incentive Stock Options,
upon the earlier of (i) the close of business on the day next preceding the
tenth anniversary of the date of its adoption by the Board of Directors, or (ii)
the date on which all shares available for issuance under the Plan shall have
been issued pursuant to the exercise or cancellation of options granted under
the Plan. Unless sooner terminated in accordance with Section 16, the Plan shall
terminate with respect to option which are not Incentive Stock Options on the
date specified in (ii) above. If the date of termination is determined under (i)
above, then options outstanding on such date shall continue to have force and
effect in accordance with the provisions of the instruments evidencing such
options.

                            Adopted by the Board of Directors on August 1, 1999.

                                       10Exhibit 4.2

                                                                 Execution copy

                         REGISTRATION RIGHTS AGREEMENT

                                  by and among

                          IMCLONE SYSTEMS INCORPORATED

                                   as Issuer,

                                      and

                       MORGAN STANLEY & CO. INCORPORATED

                                      and

               MERRILL LYNCH, PIERCE, FENNER & SMITH INCORPORATED

                             as Initial Purchasers

                         Dated as of February 29, 2000
<PAGE>

     THIS REGISTRATION RIGHTS AGREEMENT is made and entered into as of February
29, 2000, by and among ImClone Systems Incorporated, a Delaware corporation
(the "Company"), and Morgan Stanley & Co. Incorporated and Merrill Lynch,
Pierce, Fenner & Smith Incorporated (together, the "Initial Purchasers")
pursuant to the Purchase Agreement, dated February 23, 2000 (the "Purchase
Agreement") among the Company and the Initial Purchasers. In order to induce
the Initial Purchasers to enter into the Purchase Agreement, the Company has
agreed to provide the registration rights set forth in this Agreement. The
execution of this Agreement is a condition to the closing under the Purchase
Agreement.

     The Company agrees with the Initial Purchasers, (i) for their benefit as
Initial Purchasers and (ii) for the benefit of the beneficial owners (including
the Initial Purchasers) from time to time of the Notes (as defined herein) and
the beneficial owners from time to time of the Underlying Common Stock (as
defined herein) issued upon conversion of the Notes (each of the foregoing a
"Holder" and together the "Holders"), as follows:

     SECTION 1. Definitions. Capitalized terms used herein without definition
shall have their respective meanings set forth in the Purchase Agreement. As
used in this Agreement, the following terms shall have the following meanings:

     "Affiliate" means with respect to any specified person, an "affiliate," as
defined in Rule 144, of such person.

     "Amendment Effectiveness Deadline Date" has the meaning set forth in
Section 2(d) hereof.

     "Applicable Conversion Price" as of any date of determination means the
Conversion Price in effect as of such date of determination or, if no Notes are
then outstanding, the Conversion Price that would be in effect were Notes then
outstanding.

     "Business Day" means each Monday, Tuesday, Wednesday, Thursday and Friday
that is not a day on which banking institutions in The City of New York are
authorized or obligated by law or executive order to close.

     "Common Stock" means the shares of common stock, par value $.001 per
share, of the Company and any other shares of common stock as may constitute
"Common Stock" for purposes of the Indenture, including the Underlying Common
Stock.

     "Conversion Price" has the meaning assigned such term in the Indenture.

     "Damages Accrual Period" has the meaning set forth in Section 2(e) hereof.

     "Damages Payment Date" means each interest payment date under the
Indenture in the case of Notes, and each March 1 and September 1 in the case of
the Underlying Common Stock.

     "Deferral Notice" has the meaning set forth in Section 3(i) hereof.

     "Deferral Period" has the meaning set forth in Section 3(i) hereof.

     "Effectiveness Deadline Date" has the meaning set forth in Section 2(a)
hereof.

     "Effectiveness Period" means the period commencing on the date hereof and
ending on the date that all Registrable Securities have ceased to be
Registrable Securities.

     "Event" has the meaning set forth in Section 2(e) hereof.

     "Event Date" has the meaning set forth in Section 2(e) hereof.

     "Event Termination Date" has the meaning set forth in Section 2(e) hereof.

                                      -1-

<PAGE>

     "Exchange Act" means the Securities Exchange Act of 1934, as amended, and
the rules and regulations of the SEC promulgated thereunder.

     "Filing Deadline Date" has the meaning set forth in Section 2(a) hereof.

     "Holder" has the meaning set forth in the second paragraph of this
Agreement.

     "Indenture" means the Indenture, dated as of February 29, 2000, between
the Company and The Bank of New York, as trustee, pursuant to which the Notes
are being issued.

     "Initial Purchasers" means Morgan Stanley & Co. Incorporated and Merrill
Lynch, Pierce, Fenner & Smith Incorporated.

     "Initial Shelf Registration Statement" has the meaning set forth in
Section 2(a) hereof.

     "Issue Date" means the first date of original issuance of the Notes.

     "Liquidated Damages Amount" has the meaning set forth in Section 2(e)
hereof.

     "Losses" has the meaning set forth in Section 6 hereof.

     "Material Event" has the meaning set forth in Section 3(i) hereof.

     "Notes" means the 5 1/2% Convertible Subordinated Notes due 2005 of the
Company to be purchased pursuant to the Purchase Agreement.

     "Notice and Questionnaire" means a written notice delivered to the Company
containing substantially the information called for by the Selling
Securityholder Notice and Questionnaire attached as Annex B to the Offering
Memorandum of the Company issued February 23, 2000 relating to the Notes.

     "Notice Holder" means, on any date, any Holder that has delivered a Notice
and Questionnaire to the Company on or prior to such date.

     "Purchase Agreement" has the meaning set forth in the preamble hereof.

     "Prospectus" means the prospectus included in any Registration Statement
(including, without limitation, a prospectus that discloses information
previously omitted from a prospectus filed as part of an effective registration
statement in reliance upon Rule 430A promulgated under the Securities Act), as
amended or supplemented by any amendment or prospectus supplement, including
post-effective amendments, and all materials incorporated by reference or
explicitly deemed to be incorporated by reference in such Prospectus.

     "Record Holder" means (i) with respect to any Damages Payment Date
relating to any Notes as to which any such Liquidated Damages Amount has
accrued, the holder of record of such Note on the record date with respect to
the interest payment date under the Indenture on which such Damages Payment
Date shall occur and (ii) with respect to any Damages Payment Date relating to
the Underlying Common Stock as to which any such Liquidated Damages Amount has
accrued, the registered holder of such Underlying Common Stock fifteen (15)
days prior to such Damages Payment Date.

     "Registrable Securities" means the Notes until such Notes have been
converted into or exchanged for the Underlying Common Stock and, at all times
subsequent to any such conversion or exchange the Underlying Common Stock and
any securities into or for which such Underlying Common Stock has been
converted or exchanged, and any security issued with respect thereto upon any
stock dividend, split or similar event until, in the case of any such security,
(A) the earliest of (i) its effective registration under the Securities Act and
resale in accordance with the Registration Statement covering it, (ii)
expiration of the holding period that would be applicable

                                      -2-

<PAGE>

thereto under Rule 144(k) or (iii) its sale to the public pursuant to Rule 144
(or any similar provision then in force, but not Rule 144A) under the
Securities Act, and (B) as a result of the event or circumstance described in
any of the foregoing clauses (i) through (iii), the legend with respect to
transfer restrictions required under the Indenture are removed or removable in
accordance with the terms of the Indenture or such legend, as the case may be.

     "Registration Expenses" has the meaning set forth in Section 5 hereof.

     "Registration Statement" means any registration statement of the Company
that covers any of the Registrable Securities pursuant to the provisions of
this Agreement including the Prospectus, amendments and supplements to such
registration statement, including post-effective amendments, all exhibits, and
all materials incorporated by reference or explicitly deemed to be incorporated
by reference in such registration statement.

     "Restricted Securities" means "Restricted Securities" as defined in Rule
144.

     "Rule 144" means Rule 144 under the Securities Act, as such Rule may be
amended from time to time, or any similar rule or regulation hereafter adopted
by the SEC.

     "Rule 144A" means Rule 144A under the Securities Act, as such Rule may be
amended from time to time, or any similar rule or regulation hereafter adopted
by the SEC.

     "SEC" means the Securities and Exchange Commission.

     "Securities Act" means the Securities Act of 1933, as amended, and the
rules and regulations promulgated by the SEC thereunder.

     "Shelf Registration Statement" has the meaning set forth in Section 2(a)
hereof.

     "Special Counsel" means Ropes & Gray or such other successor counsel as
shall be specified by the Holders of a majority of the Registrable Securities,
but which may, with the written consent of the Initial Purchasers (which shall
not be unreasonably withheld), be another nationally recognized law firm
experienced in securities law matters designated by the Company, the reasonable
fees and expenses of which will be paid by the Company pursuant to Section 5
hereof.

     "Subsequent Shelf Registration Statement" has the meaning set forth in
Section 2(b) hereof.

     "TIA" means the Trust Indenture Act of 1939, as amended.

     "Trustee" means The Bank of New York, the Trustee under the Indenture.

     "Underlying Common Stock" means the Common Stock into which the Notes are
convertible or issued upon any such conversion.

     SECTION 2. Shelf Registration. (a) The Company shall use its reasonable
efforts to prepare and file or cause to be prepared and filed with the SEC, as
soon as practicable but in any event by the date (the "Filing Deadline Date")
ninety (90) days after the Issue Date, a Registration Statement for an offering
to be made on a delayed or continuous basis pursuant to Rule 415 of the
Securities Act (a "Shelf Registration Statement") registering the resale from
time to time by Holders thereof of all of the Registrable Securities (the
"Initial Shelf Registration Statement"). The Initial Shelf Registration
Statement shall be on Form S-3 or another appropriate form permitting
registration of such Registrable Securities for resale by such Holders in
accordance with the methods of distribution elected by the Holders and set
forth in the Initial Shelf Registration Statement. The Company shall use its
reasonable efforts to cause the Initial Shelf Registration Statement to be
declared effective under the Securities Act as promptly as is practicable but
in any event by the date (the "Effectiveness Deadline Date") that is one
hundred eighty (180) days after the Issue Date, and to keep the Initial Shelf
Registration Statement (or any Subsequent Shelf Registration Statement, as
defined below) continuously effective under the Securities Act until the
expiration of the Effectiveness

                                      -3-

<PAGE>

Period. At the time the Initial Shelf Registration Statement is declared
effective, each Holder that became a Notice Holder on or prior to the date ten
(10) Business Days prior to such time of effectiveness shall be named as a
selling securityholder in the Initial Shelf Registration Statement and the
related Prospectus in such a manner as to permit such Holder to deliver such
Prospectus to purchasers of Registrable Securities in accordance with
applicable law. Except for the piggyback registration rights granted to Merck
KGaA under its Development and License Agreement dated December 14, 1998 with
the Company for which the Company will use its reasonable efforts to obtain a
waiver or offer Merck KGaA a separate shelf registration, none of the Company's
security holders (other than the Holders of Registrable Securities) shall have
the right to include the Company's securities in the Shelf Registration
Statement.

         (b) If the Initial Shelf Registration Statement or any Subsequent
Shelf Registration Statement ceases to be effective for any reason at any time
during the Effectiveness Period (other than because all Registrable Securities
registered thereunder shall have been resold pursuant thereto or shall have
otherwise ceased to be Registrable Securities), the Company shall use its
reasonable efforts to obtain the prompt withdrawal of any order suspending the
effectiveness thereof, and in any event shall within thirty (30) days of such
cessation of effectiveness amend the Shelf Registration Statement in a manner
reasonably expected to obtain the withdrawal of the order suspending the
effectiveness thereof, or file an additional Shelf Registration Statement
covering all of the securities that as of the date of such filing are
Registrable Securities (a "Subsequent Shelf Registration Statement"). If a
Subsequent Shelf Registration Statement is filed, the Company shall use its
reasonable efforts to cause the Subsequent Shelf Registration Statement to
become effective as promptly as is practicable after such filing and to keep
such Registration Statement (or subsequent Shelf Registration Statement)
continuously effective until the end of the Effectiveness Period.

         (c) The Company shall supplement and amend the Shelf Registration
Statement if required by the rules, regulations or instructions applicable to
the registration form used by the Company for such Shelf Registration
Statement, if required by the Securities Act or as reasonably requested by the
Initial Purchasers or by the Trustee on behalf of the Holders of the
Registrable Securities covered by such Shelf Registration Statement.

         (d) Each Holder of Registrable Securities agrees that if such Holder
wishes to sell Registrable Securities pursuant to a Shelf Registration
Statement and related Prospectus, it will do so only in accordance with this
Section 2(d) and Section 3(i). Each Holder of Registrable Securities wishing to
sell Registrable Securities pursuant to a Shelf Registration Statement and
related Prospectus agrees to deliver a Notice and Questionnaire to the Company
at least three (3) Business Days prior to any intended distribution of
Registrable Securities under the Shelf Registration Statement. From and after
the date the Initial Shelf Registration Statement is declared effective, the
Company shall, as promptly as practicable after the date a Notice and
Questionnaire is delivered, and in any event upon the later of (x) five (5)
Business Days after such date or (y) five (5) Business Days after the
expiration of any Deferral Period in effect when the Notice and Questionnaire
is delivered, (i) if required by applicable law, file with the SEC a
post-effective amendment to the Shelf Registration Statement or prepare and, if
required by applicable law, file a supplement to the related Prospectus or a
supplement or amendment to any document incorporated therein by reference or
file any other required document so that the Holder delivering such Notice and
Questionnaire is named as a selling securityholder in the Shelf Registration
Statement and the related Prospectus in such a manner as to permit such Holder
to deliver such Prospectus to purchasers of the Registrable Securities in
accordance with applicable law and, if the Company shall file a post-effective
amendment to the Shelf Registration Statement, use its reasonable efforts to
cause such post-effective amendment to be declared effective under the
Securities Act as promptly as is practicable, but in any event by the date (the
"Amendment Effectiveness Deadline Date") that is forty-five (45) days after the
date such post-effective amendment is required by this clause to be filed; (ii)
provide such Holder copies of any documents filed pursuant to Section 2(d)(i);
and (iii) notify such Holder as promptly as practicable after the effectiveness
under the Securities Act of any post-effective amendment filed pursuant to
Section 2(d)(i); provided, that if such Notice and Questionnaire is delivered
during a Deferral Period, the Company shall so inform the Holder delivering
such Notice and Questionnaire and shall take the actions set forth in clauses
(i), (ii) and (iii) above upon expiration of the Deferral Period in accordance
with Section 3(i). Notwithstanding anything contained herein to the contrary,
(i) the Company shall be under no obligation to name any Holder that is not a
Notice Holder as a selling securityholder in any Registration Statement or
related Prospectus and (ii) the Amendment Effectiveness Deadline Date shall be
extended by up to ten (10) Business Days from the expiration of a

                                      -4-

<PAGE>

Deferral Period (and the Company shall incur no obligation to pay Liquidated
Damages during such extension) if such Deferral Period shall be in effect on
the Amendment Effectiveness Deadline Date.

         (e) The parties hereto agree that the Holders of Registrable
Securities will suffer damages, and that it would not be feasible to ascertain
the extent of such damages with precision, if (i) the Initial Shelf
Registration Statement has not been filed on or prior to the Filing Deadline
Date, (ii) the Initial Shelf Registration Statement has not been declared
effective under the Securities Act on or prior to the Effectiveness Deadline
Date, (iii) the Company has failed to perform its obligations set forth in
Section 2(d) within the time period required therein, (iv) the aggregate
duration of Deferral Periods in any period exceeds the number of days permitted
in respect of such period pursuant to Section 3(i) hereof or (v) the number of
Deferral Periods in any period exceeds the number permitted in respect of such
period pursuant to Section 3(i) hereof (each of the events of a type described
in any of the foregoing clauses (i) through (v) are individually referred to
herein as an "Event," and the Filing Deadline Date in the case of clause (i),
the Effectiveness Deadline Date in the case of clause (ii), the date by which
the Company is required to perform its obligations set forth in Section 2(d) in
the case of clause (iii) (including the filing of any post-effective amendment
prior to the Amendment Effectiveness Deadline Date), the date on which the
aggregate duration of Deferral Periods in any period exceeds the number of days
permitted by Section 3(i) hereof in the case of clause (iv), and the date of
the commencement of a Deferral Period that causes the limit on the number of
Deferral Periods in any period under Section 3(i) hereof to be exceeded in the
case of clause (v), being referred to herein as an "Event Date"). Events shall
be deemed to continue until the "Event Termination Date," which shall be the
following dates with respect to the respective types of Events: the date the
Initial Shelf Registration Statement is filed in the case of an Event of the
type described in clause (i), the date the Initial Shelf Registration Statement
is declared effective under the Securities Act in the case of an Event of the
type described in clause (ii), the date the Company performs its obligations
set forth in Section 2(d) in the case of an Event of the type described in
clause (iii) (including, without limitation, the date the relevant
post-effective amendment to the Shelf Registration Statement is declared
effective under the Securities Act), termination of the Deferral Period that
caused the limit on the aggregate duration of Deferral Periods in a period set
forth in Section 3(i) to be exceeded in the case of the commencement of an
Event of the type described in clause (iv), and termination of the Deferral
Period the commencement of which caused the number of Deferral Periods in a
period permitted by Section 3(i) to be exceeded in the case of an Event of the
type described in clause (v).

     Accordingly, commencing on (and including) any Event Date and ending on
(but excluding) the next date on which there are no Events that have occurred
and are continuing (a "Damages Accrual Period"), the Company agrees to pay, as
liquidated damages and not as a penalty, an amount (the "Liquidated Damages
Amount"), payable on the Damages Payment Dates to Record Holders of Notes that
are Registrable Securities and of shares of Underlying Common Stock issued upon
conversion of Notes that are Registrable Securities, as the case may be,
accruing, for each portion of such Damages Accrual Period beginning on and
including a Damages Payment Date (or, in respect of the first time that the
Liquidated Damages Amount is to be paid to Holders on a Damages Payment Date as
a result of the occurrence of any particular Event, from the Event Date) and
ending on but excluding the first to occur of (A) the date of the end of the
Damages Accrual Period or (B) the next Damages Payment Date, at a rate per
annum equal to one-half of one percent (0.5 %) of the aggregate principal
amount of such Notes or, in the case of Notes that have been converted into or
exchanged for Underlying Common Stock, the Applicable Conversion Price of such
shares of Underlying Common Stock, as the case may be, in each case determined
as of the Business Day immediately preceding the next Damages Payment Date;
provided, that in the case of a Damages Accrual Period that is in effect solely
as a result of an Event of the type described in clause (iii) of the
immediately preceding paragraph, such Liquidated Damages Amount shall be paid
only to the Holders that have delivered Notice and Questionnaires that caused
the Company to incur the obligations set forth in Section 2(d) the
non-performance of which is the basis of such Event, provided further, that any
Liquidated Damages Amount accrued with respect to any Note or portion thereof
called for redemption on a redemption date or converted into Underlying Common
Stock on a conversion date prior to the Damages Payment Date, shall, in any
such event, be paid instead to the Holder who submitted such Note or portion
thereof for redemption or conversion on the applicable redemption date or
conversion date, as the case may be, on such date (or promptly following the
conversion date, in the case of conversion). Notwithstanding the foregoing, no
Liquidated Damages Amounts shall accrue as to any Registrable Security from and
after the earlier of (x) the date such security is no longer a Registrable
Security and (y) expiration of the Effectiveness Period. The rate of accrual of
the Liquidated Damages Amount with respect to any period shall

                                      -5-

<PAGE>

not exceed the rate provided for in this paragraph notwithstanding the
occurrence of multiple concurrent Events. Following the cure of all Events
requiring the payment by the Company of Liquidated Damages Amounts to the
Holders of Registrable Securities pursuant to this Section, the accrual of
Liquidated Damages Amounts will cease (without in any way limiting the effect
of any subsequent Event requiring the payment of Liquidated Damages Amount by
the Company).

     The Trustee shall be entitled, on behalf of Holders of Notes or Underlying
Common Stock, to seek any available remedy for the enforcement of this
Agreement, including for the payment of any Liquidated Damages Amount.
Notwithstanding the foregoing, the parties agree that the sole damages payable
for a violation of the terms of this Agreement with respect to which liquidated
damages are expressly provided shall be such liquidated damages. Nothing shall
preclude a Notice Holder or Holder of Registrable Securities from pursuing or
obtaining specific performance or other equitable relief with respect to this
Agreement.

     All of the Company's obligations set forth in this Section 2(e) that are
outstanding with respect to any Registrable Security at the time such security
ceases to be a Registrable Security shall survive until such time as all such
obligations with respect to such security have been satisfied in full
(notwithstanding termination of this Agreement pursuant to Section 8(k)).

     The parties hereto agree that the liquidated damages provided for in this
Section 2(e) constitute a reasonable estimate of the damages that may be
incurred by Holders of Registrable Securities by reason of the failure of the
Shelf Registration Statement to be filed or declared effective or available for
effecting resales of Registrable Securities in accordance with the provisions
hereof.

     SECTION 3. Registration Procedures. In connection with the registration
obligations of the Company under Section 2 hereof, the Company shall:

         (a) Prepare and file with the SEC a Registration Statement or
Registration Statements on any appropriate form under the Securities Act
available for the sale of the Registrable Securities by the Holders thereof in
accordance with the intended method or methods of distribution thereof, and use
its reasonable efforts to cause each such Registration Statement to become
effective and remain effective as provided herein; provided, that before filing
any Registration Statement or Prospectus or any amendments or supplements
thereto with the SEC, furnish to the Initial Purchasers and the Special Counsel
copies of all such documents proposed to be filed and use its reasonable
efforts to reflect in each such document when so filed with the SEC such
comments as the Special Counsel reasonably shall propose within five (5)
Business Days of the delivery of such copies to the Initial Purchasers and the
Special Counsel.

         (b) Prepare and file with the SEC such amendments and post-effective
amendments to each Registration Statement as may be necessary to keep such
Registration Statement continuously effective for the applicable period
specified in Section 2(a); cause the related Prospectus to be supplemented by
any required Prospectus supplement, and as so supplemented to be filed pursuant
to Rule 424 (or any similar provisions then in force) under the Securities Act;
and use its reasonable efforts to comply with the provisions of the Securities
Act applicable to it with respect to the disposition of all securities covered
by such Registration Statement during the Effectiveness Period in accordance
with the intended methods of disposition by the sellers thereof set forth in
such Registration Statement as so amended or such Prospectus as so
supplemented.

         (c) As promptly as practicable give notice to the Notice Holders, the
Initial Purchasers and the Special Counsel (i) when any Prospectus, Prospectus
supplement, Registration Statement or post-effective amendment to a
Registration Statement has been filed with the SEC and, with respect to a
Registration Statement or any post-effective amendment, when the same has been
declared effective, (ii) of any request, following the effectiveness of the
Initial Shelf Registration Statement under the Securities Act, by the SEC or
any other federal or state governmental authority for amendments or supplements
to any Registration Statement or related Prospectus or for additional
information, (iii) of the issuance by the SEC or any other federal or state
governmental authority of any stop order suspending the effectiveness of any
Registration Statement or the initiation or threatening of any proceedings for
that purpose, (iv) of the receipt by the Company of any notification with
respect to the suspension of

                                      -6-

<PAGE>

the qualification or exemption from qualification of any of the Registrable
Securities for sale in any jurisdiction or the initiation or threatening of any
proceeding for such purpose, (v) of the occurrence of (but not the nature of or
details concerning) a Material Event and (vi) of the determination by the
Company that a post-effective amendment to a Registration Statement will be
filed with the SEC, which notice may, at the discretion of the Company (or as
required pursuant to Section 3 (i)), state that it constitutes a Deferral
Notice, in which event the provisions of Section 3(i) shall apply.

         (d) Use reasonable efforts to obtain the withdrawal of any order
suspending the effectiveness of a Registration Statement or the lifting of any
suspension of the qualification (or exemption from qualification) of any of the
Registrable Securities for sale in any jurisdiction in which they have been
qualified for sale, in either case at the earliest possible moment, and provide
prompt notice to each Notice Holder and the Initial Purchasers of the
withdrawal of any such order.

         (e) If reasonably requested by the Initial Purchasers or any Notice
Holder, as promptly as practicable incorporate in a Prospectus supplement or
post-effective amendment to a Registration Statement such information as the
Initial Purchasers, the Special Counsel or such Notice Holder shall, on the
basis of a written opinion of nationally-recognized counsel experienced in such
matters, determine to be required to be included therein by applicable law and
make any required filings of such Prospectus supplement or such post-effective
amendment.

         (f) As promptly as practicable furnish to each Notice Holder, the
Special Counsel and the Initial Purchasers, without charge, at least one (1)
conformed copy of the Registration Statement and any amendment thereto,
including financial statements but excluding schedules, all documents
incorporated or deemed to be incorporated therein by reference and all exhibits
(unless requested in writing to the Company by such Notice Holder, Special
Counsel, counsel or Initial Purchasers).

         (g) During the Effectiveness Period, deliver to each Notice Holder,
the Special Counsel and the Initial Purchasers, in connection with any sale of
Registrable Securities pursuant to a Registration Statement, without charge, as
many copies of the Prospectus or Prospectuses relating to such Registrable
Securities (including each preliminary prospectus) and any amendment or
supplement thereto as such Notice Holder may reasonably request; and the
Company hereby consents (except during such periods that a Deferral Notice is
outstanding and has not been revoked) to the use of such Prospectus or each
amendment or supplement thereto by each Notice Holder, in connection with any
offering and sale of the Registrable Securities covered by such Prospectus or
any amendment or supplement thereto in the manner set forth therein.

         (h) Prior to any public offering of the Registrable Securities
pursuant to the Shelf Registration Statement, register or qualify or cooperate
with the Notice Holders in connection with the registration or qualification
(or exemption from such registration or qualification) of such Registrable
Securities for offer and sale under the securities or Blue Sky laws of such
jurisdictions within the United States as any Notice Holder reasonably requests
in writing (which request may be included in the Notice and Questionnaire);
prior to any public offering of the Registrable Securities pursuant to the
Shelf Registration Statement, keep each such registration or qualification (or
exemption therefrom) effective during the Effectiveness Period in connection
with such Notice Holder's offer and sale of Registrable Securities pursuant to
such registration or qualification (or exemption therefrom) and do any and all
other acts or things reasonably necessary or advisable to enable the
disposition in such jurisdictions of such Registrable Securities in the manner
set forth in the relevant Registration Statement and the related Prospectus;
provided, that the Company will not be required to (i) qualify as a foreign
corporation or as a dealer in securities in any jurisdiction where it would not
otherwise be required to qualify but for this Agreement or (ii) take any action
that would subject it to general service of process in suits or to taxation in
any such jurisdiction where it is not then so subject.

         (i) Upon (A) the issuance by the SEC of a stop order suspending the
effectiveness of the Shelf Registration Statement or the initiation of
proceedings with respect to the Shelf Registration Statement under Section 8(d)
or 8(e) of the Securities Act, (B) the occurrence of any event or the existence
of any fact (a "Material Event") as a result of which any Registration
Statement shall 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 not misleading, or any

                                      -7-

<PAGE>

Prospectus shall 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, not misleading, or (C) the occurrence or existence of any pending
corporate development that, in the reasonable discretion of the Company, makes
it appropriate to suspend the availability of the Shelf Registration Statement
and the related Prospectus, (i) in the case of clause (B) above, subject to the
next sentence, as promptly as practicable prepare and file, if necessary
pursuant to applicable law, a post-effective amendment to such Registration
Statement or a supplement to the related Prospectus or any document
incorporated therein by reference or file any other required document that
would be incorporated by reference into such Registration Statement and
Prospectus so that such Registration Statement does 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 not misleading
(except for any such untrue statement or omission made in reliance on and in
conformity with information relating to any Notice Holder furnished to the
Company in writing by such Notice Holder expressly for use therein), and such
Prospectus does 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, not misleading (except for any such untrue statement or omission made in
reliance on and in conformity with information relating to any Notice Holder
furnished to the Company in writing by such Notice Holder expressly for use
therein), as thereafter delivered to the purchasers of the Registrable
Securities being sold thereunder, and, in the case of a post-effective
amendment to a Registration Statement, subject to the next sentence, use its
reasonable efforts to cause it to be declared effective as promptly as is
practicable, and (ii) give notice to the Notice Holders and the Special Counsel
that the availability of the Shelf Registration Statement is suspended (a
"Deferral Notice") and, upon receipt of any Deferral Notice, each Notice Holder
agrees not to sell any Registrable Securities pursuant to the Registration
Statement until such Notice Holder's receipt of copies of the supplemented or
amended Prospectus provided for in clause (i) above, or until it is advised in
writing by the Company that the Prospectus may be used, and has received copies
of any additional or supplemental filings that are incorporated or deemed
incorporated by reference in such Prospectus. The Company will use all
reasonable efforts to ensure that the use of the Prospectus may be resumed (x)
in the case of clause (A) above, as promptly as is practicable, (y) in the case
of clause (B) above, as soon as, in the sole judgment of the Company, public
disclosure of such Material Event would not be prejudicial to or contrary to
the interests of the Company or, if necessary to avoid unreasonable burden or
expense, as soon as practicable thereafter and (z) in the case of clause (C)
above, as soon as, in the discretion of the Company, such suspension is no
longer appropriate. The Company shall be entitled to exercise its right under
this Section 3(i) to suspend the availability of the Shelf Registration
Statement or any Prospectus, without incurring or accruing any obligation to
pay liquidated damages pursuant to Section 2(e), no more than one (1) time in
any three month period or three (3) times in any twelve month period, and any
such period during which the availability of the Registration Statement and any
Prospectus is suspended (the "Deferral Period") shall, without incurring any
obligation to pay liquidated damages pursuant to Section 2(e), not exceed 30
days; provided, that in the case of a Material Event relating to an acquisition
or a probable acquisition or financing, recapitalization, business combination
or other similar transaction, the Company may, without incurring any obligation
to pay liquidated damages pursuant to Section 2(e), deliver to Notice Holders a
second notice to the effect set forth above, which shall have the effect of
extending the Deferral Period by up to an additional 30 days, or such shorter
period of time as is specified in such second notice, provided, that the
aggregate duration of any Deferral Periods shall not, without incurring any
obligation to pay liquidated damages pursuant to Section 2(e), exceed 30 days
in any three month period (or 60 days in any three month period in the event of
a Material Event pursuant to which the Company has delivered a second notice as
required above) or 90 days in any twelve (12) month period.

         (j) If requested in writing in connection with a disposition of
Registrable Securities pursuant to a Registration Statement, make reasonably
available for inspection during normal business hours by a representative for
the Notice Holders of such Registrable Securities, and any managing
underwriter, broker-dealers, attorneys and accountants retained by such Notice
Holders, all relevant financial and other records and pertinent corporate
documents and properties of the Company and its subsidiaries, and cause the
appropriate officers, directors and employees of the Company and its
subsidiaries to make reasonably available for inspection during normal business
hours on reasonable notice all relevant information reasonably requested by
such representative for the Notice Holders, or any such managing underwriter,
broker-dealers, attorneys or accountants in connection with such disposition,
in each case as is customary for similar "due diligence" examinations;
provided, however, that such persons shall first agree in writing with the
Company that any information that is reasonably and in good faith

                                      -8-

<PAGE>

designated by the Company in writing as confidential at the time of delivery of
such information shall be kept confidential by such persons and shall be used
solely for the purposes of exercising rights under this Agreement, unless (i)
disclosure of such information is required by court or administrative order or
is necessary to respond to inquiries of regulatory authorities, (ii) disclosure
of such information is required by law (including any disclosure requirements
pursuant to federal securities laws in connection with the filing of any
Registration Statement or the use of any Prospectus referred to in this
Agreement), (iii) such information becomes generally available to the public
other than as a result of a disclosure or failure to safeguard by any such
person or (iv) such information becomes available to any such person from a
source other than the Company and such source is not bound by a confidentiality
agreement, and provided, that the foregoing inspection and information
gathering shall, to the greatest extent possible, be coordinated on behalf of
all the Notice Holders and the other parties entitled thereto by the counsel
referred to in Section 5 and provided further, that the Company shall not be
required to disclose any information subject to the attorney-client or attorney
work product privilege if and to the extent such disclosure would constitute a
waiver of such privilege.

         (k) Use all reasonable efforts to comply with all applicable rules and
regulations of the SEC and make generally available to its securityholders
earning statements (which need not be audited) satisfying the provisions of
Section 11(a) of the Securities Act and Rule 158 thereunder (or any similar
rule promulgated under the Securities Act) no later than 45 days after the end
of any 3-month period (or 90 days after the end of any 12-month period if such
period is a fiscal year) commencing on the first day of the first fiscal
quarter of the Company commencing after the effective date of a Registration
Statement, which statements shall cover said periods.

         (l) Cooperate with each Notice Holder to facilitate the timely
preparation and delivery of certificates representing Registrable Securities
sold or to be sold pursuant to a Registration Statement, which certificates
shall not bear any restrictive legends, and cause such Registrable Securities
to be in such denominations as are permitted by the Indenture and registered in
such names as such Notice Holder may request in writing at least two (2)
Business Days prior to any sale of such Registrable Securities.

         (m) Provide a CUSIP number for all Registrable Securities covered by
each Registration Statement not later than the effective date of such
Registration Statement and provide the Trustee and the transfer agent for the
Common Stock with printed certificates for the Registrable Securities that are
in a form eligible for deposit with The Depository Trust Company.

         (n) Cooperate and assist in any filings required to be made with the
National Association of Securities Dealers, Inc.

         (o) Upon (i) the filing of the Initial Registration Statement and (ii)
the effectiveness of the Initial Registration Statement, announce the same, in
each case by release to Reuters Economic Services and Bloomberg Business News.

     SECTION 4. Holder's Obligations. Each Holder agrees, by acquisition of the
Registrable Securities, that no Holder of Registrable Securities shall be
entitled to sell any of such Registrable Securities pursuant to a Registration
Statement or to receive a Prospectus relating thereto, unless such Holder has
furnished the Company with a Notice and Questionnaire as required pursuant to
Section 2(d) hereof (including the information required to be included in such
Notice and Questionnaire) and the information set forth in the next sentence.
Each Notice Holder agrees promptly to furnish to the Company all information
required to be disclosed in order to make the information previously furnished
to the Company by such Notice Holder not misleading and any other information
regarding such Notice Holder and the distribution of such Registrable
Securities as the Company may from time to time reasonably request. Any sale of
any Registrable Securities by any Holder shall constitute a representation and
warranty by such Holder that the information relating to such Holder and its
plan of distribution is as set forth in the Prospectus delivered by such Holder
in connection with such disposition, that such Prospectus does not as of the
time of such sale contain any untrue statement of a material fact relating to
or provided by such Holder or its plan of distribution and that such Prospectus
does not as of the time of such sale omit to state any material fact relating
to or provided by such Holder or its plan of distribution necessary to make the
statements in such Prospectus, in the light of the circumstances under which
they were made, not misleading.

                                      -9-

<PAGE>

     SECTION 5. Registration Expenses. The Company shall bear all fees and
expenses incurred in connection with the performance by the Company of its
obligations under Sections 2 and 3 of this Agreement whether or not any of the
Registration Statements are declared effective. Such fees and expenses shall
include, without limitation, (i) all registration and filing fees (including,
without limitation, fees and expenses (x) with respect to filings required to
be made with the National Association of Securities Dealers, Inc. and (y) of
compliance with federal and state securities or Blue Sky laws (including,
without limitation, reasonable fees and disbursements of the Special Counsel in
connection with Blue Sky qualifications of the Registrable Securities under the
laws of such jurisdictions as the Notice Holders of a majority of the
Registrable Securities being sold pursuant to a Registration Statement may
designate), (ii) printing expenses (including, without limitation, expenses of
printing certificates for Registrable Securities in a form eligible for deposit
with The Depository Trust Company, (iii) duplication expenses relating to
copies of any Registration Statement or Prospectus delivered to any Holders
hereunder, (iv) fees and disbursements of counsel for the Company and the
Special Counsel in connection with the Shelf Registration Statement (provided
that the Company shall not be liable for the fees and expenses of more than one
separate firm for all parties participating in any transaction hereunder), (v)
reasonable fees and disbursements of the Trustee and its counsel and of the
registrar and transfer agent for the Common Stock and (vi) Securities Act
liability insurance obtained by the Company in its sole discretion. In
addition, the Company shall pay the internal expenses of the Company
(including, without limitation, all salaries and expenses of officers and
employees performing legal or accounting duties), the expense of any annual
audit, the fees and expenses incurred in connection with the listing by the
Company of the Registrable Securities on any securities exchange on which
similar securities of the Company are then listed and the fees and expenses of
any person, including special experts, retained by the Company. Notwithstanding
the provisions of this Section 5, each seller of Registrable Securities shall
pay selling expenses and all registration expenses to the extent required by
applicable law.

     SECTION 6.  Indemnification.

         (a) Indemnification by the Company. The Company shall indemnify and
hold harmless each Notice Holder and each person, if any, who controls any
Notice Holder (within the meaning of either Section 15 of the Securities Act or
Section 20 of the Exchange Act) from and against any losses, liabilities,
claims, damages and expenses (including, without limitation, any legal or other
expenses reasonably incurred in connection with defending or investigating any
such action or claim) (collectively, "Losses"), arising out of or based upon
any untrue statement or alleged untrue statement of a material fact contained
in any Registration Statement or Prospectus or in any amendment or supplement
thereto or in any preliminary prospectus, or arising out of or based upon any
omission or alleged omission to state therein a material fact required to be
stated therein or necessary to make the statements therein not misleading,
provided, however, that the Company shall not be liable in any such case to the
extent that any such Losses arise out of or are based upon an untrue statement
or alleged untrue statement contained in or omission or alleged omission from
any of such documents in reliance upon and conformity with any of the
information relating to the Holders furnished to the Company in writing by a
Holder expressly for use therein; provided further, that the indemnification
contained in this paragraph shall not inure to the benefit of any Holder of
Registrable Securities (or to the benefit of any person controlling such
Holder) on account of any such Losses arising out of or based upon an untrue
statement or alleged untrue statement or omission or alleged omission made in
any preliminary prospectus provided in each case the Company has performed its
obligations under Section 3(a) hereof if either (A) (i) such Holder failed to
send or deliver a copy of the Prospectus with or prior to the delivery of
written confirmation of the sale by such Holder to the person asserting the
claim from which such Losses arise and (ii) the Prospectus would have corrected
such untrue statement or alleged untrue statement or such omission or alleged
omission, or (B) (x) such untrue statement or alleged untrue statement,
omission or alleged omission is corrected in an amendment or supplement to the
Prospectus and (y) having previously been furnished by or on behalf of the
Company with copies of the Prospectus as so amended or supplemented, such
Holder thereafter fails to deliver such Prospectus as so amended or
supplemented, with or prior to the delivery of written confirmation of the sale
of a Registrable Security to the person asserting the claim from which such
Losses arise.

         (b) Indemnification by Holders of Registrable Securities. Each Holder
agrees severally and not jointly to indemnify and hold harmless the Company and
its respective directors and officers, and each person, if any, who controls
the Company (within the meaning of either Section 15 of the Securities Act or
Section 20 of the Exchange Act) or any other Holder, from and against all
Losses arising out of or based upon any untrue statement or alleged

                                      -10-

<PAGE>

untrue statement of a material fact contained in any Registration Statement or
Prospectus or in any amendment or supplement thereto or in any preliminary
prospectus, or arising out of or based upon any omission or alleged omission to
state therein a material fact required to be stated therein or necessary to
make the statements therein not misleading, to the extent, but only to the
extent, that such untrue statement or alleged untrue statement or omission or
alleged omission was made in reliance upon and in conformity with information
furnished to the Company by such Holder expressly for use in such Registration
Statement or Prospectus or amendment or supplement thereto. In no event shall
the liability of any selling Holder of Registrable Securities hereunder be
greater in amount than the dollar amount of the proceeds received by such
Holder upon the sale of the Registrable Securities pursuant to the Registration
Statement giving rise to such indemnification obligation.

         (c) Conduct of Indemnification Proceedings. In case any proceeding
(including any governmental investigation) shall be instituted involving any
person in respect of which indemnity may be sought pursuant to either of the
two preceding paragraphs, such person (the "indemnified party") shall promptly
notify the person against whom such indemnity may be sought (the "indemnifying
party") in writing and the indemnifying party, upon request of the indemnified
party, shall retain counsel reasonably satisfactory to the indemnified party to
represent the indemnified party and any others the indemnifying party may
designate in such proceeding and shall pay the reasonable fees and
disbursements of such counsel related to such proceeding. In any such
proceeding, any indemnified party shall have the right to retain its own
counsel, but the fees and expenses of such counsel shall be at the expense of
such indemnified party unless (i) the indemnifying party and the indemnified
party shall have mutually agreed to the retention of such counsel or (ii) the
named parties to any such proceeding (including any impleaded parties) include
both the indemnifying party and the indemnified party and representation of
both parties by the same counsel would be inappropriate due to actual or
potential differing interests between them. It is understood that the
indemnifying party shall not, in respect of the legal expenses of any
indemnified party in connection with any proceeding or related proceedings in
the same jurisdiction, be liable for the fees and expenses of more than one
separate firm (in addition to any local counsel) for all indemnified parties,
and that all such fees and expenses shall be reimbursed as they are incurred.
Such separate firm shall be designated in writing by, in the case of parties
indemnified pursuant to Section 6(a), the Holders of a majority (with Holders
of Notes deemed to be the Holders, for purposes of determining such majority,
of the number of shares of Underlying Common Stock into which such Notes are or
would be convertible or exchangeable as of the date on which such designation
is made) of the Registrable Securities covered by the Registration Statement
held by Holders that are indemnified parties pursuant to Section 6(a) and, in
the case of parties indemnified pursuant to Section 6(b), the Company. The
indemnifying party shall not be liable for any settlement of any proceeding
effected without its written consent, but if settled with such consent or if
there be a final judgment for the plaintiff, the indemnifying party agrees to
indemnify the indemnified party from and against any loss or liability by
reason of such settlement or judgment. No indemnifying party shall, without the
prior written consent of the indemnified party, effect any settlement of any
pending or threatened proceeding in respect of which any indemnified party is
or could have been a party and indemnity could have been sought hereunder by
such indemnified party, unless such settlement includes an unconditional
release of such indemnified party from all liability on claims that are the
subject matter of such proceeding.

         (d) Contribution. To the extent that the indemnification provided for
in this Section 6 is unavailable to an indemnified party under Section 6(a) or
6(b) hereof in respect of any Losses or is insufficient to hold such
indemnified party harmless, then each applicable indemnifying party, in lieu of
indemnifying such indemnified party, shall contribute to the amount paid or
payable by such indemnified party as a result of such Losses (i) in such
proportion as is appropriate to reflect the relative benefits received by the
indemnifying party or parties on the one hand and the indemnified party or
parties on the other hand or (ii) if the allocation provided by clause (i)
above is not permitted by applicable law, in such proportion as is appropriate
to reflect not only the relative benefits referred to in clause (i) above but
also the relative fault of the indemnifying party or parties on the one hand
and of the indemnified party or parties on the other hand in connection with
the statements or omissions that resulted in such Losses, as well as any other
relevant equitable considerations. Benefits received by the Company shall be
deemed to be equal to the total net proceeds from the initial placement
pursuant to the Purchase Agreement (before deducting expenses) of the
Registrable Securities to which such Losses relate. Benefits received by any
Holder shall be deemed to be equal to the value of receiving Registrable
Securities that are registered under the Securities Act. The relative fault of
the Holders on the one hand and the Company on the other hand shall be
determined by

                                      -11-

<PAGE>

reference to, among other things, whether the untrue or alleged untrue
statement of a material fact or the omission or alleged omission to state a
material fact relates to information supplied by the Holders or by the Company,
and the parties' relative intent, knowledge, access to information and
opportunity to correct or prevent such statement or omission. The Holders'
respective obligations to contribute pursuant to this paragraph are several in
proportion to the respective number of Registrable Securities they have sold
pursuant to a Registration Statement, and not joint.

     The parties hereto agree that it would not be just and equitable if
contribution pursuant to this Section 6(d) were determined by pro rata
allocation or by any other method or allocation that does not take into account
the equitable considerations referred to in the immediately preceding
paragraph. The amount paid or payable by an indemnified party as a result of
the Losses referred to in the immediately preceding paragraph shall be deemed
to include, subject to the limitations set forth above, any legal or other
expenses reasonably incurred by such indemnified party in connection with
investigating or defending any such action or claim. Notwithstanding this
Section 6(d), an indemnifying party that is a selling Holder of Registrable
Securities shall not be required to contribute any amount in excess of the
amount by which the total price at which the Registrable Securities sold by
such indemnifying party and distributed to the public were offered to the
public exceeds the amount of any damages that such indemnifying party has
otherwise been required to pay by reason of such untrue or alleged untrue
statement or omission or alleged omission. No person guilty of fraudulent
misrepresentation (within the meaning of Section 11(f) of the Securities Act)
shall be entitled to contribution from any person who was not guilty of such
fraudulent misrepresentation.

         (e) The indemnity, contribution and expense reimbursement obligations
of the parties hereunder shall be in addition to any liability any indemnified
party may otherwise have hereunder, under the Purchase Agreement or otherwise.

         (f) The indemnity and contribution provisions contained in this
Section 6 shall remain operative and in full force and effect regardless of (i)
any termination of this Agreement, (ii) any investigation made by or on behalf
of any Holder or any person controlling any Holder, or the Company, or the
Company's officers or directors or any person controlling the Company and (iii)
the sale of any Registrable Securities by any Holder.

     SECTION 7. Information Requirements. (a) The Company covenants that, if at
any time before the end of the Effectiveness Period the Company is not subject
to the reporting requirements of the Exchange Act, it will cooperate with any
Holder of Registrable Securities and take such further reasonable action as any
Holder of Registrable Securities may reasonably request in writing (including,
without limitation, making such reasonable representations as any such Holder
may reasonably request), all to the extent required from time to time to enable
such Holder to sell Registrable Securities without registration under the
Securities Act within the limitation of the exemptions provided by Rule 144 and
Rule 144A under the Securities Act and customarily taken in connection with
sales pursuant to such exemptions. Upon the written request of any Holder of
Registrable Securities, the Company shall deliver to such Holder a written
statement as to whether it has complied with such filing requirements, unless
such a statement has been included in the Company's most recent report filed
pursuant to Section 13 or Section 15(d) of Exchange Act. Notwithstanding the
foregoing, nothing in this Section 7 shall be deemed to require the Company to
register any of its securities (other than the Common Stock) under any section
of the Exchange Act.

         (b) The Company shall file the reports required to be filed by it
under the Exchange Act and shall comply with all other requirements set forth
in the instructions to Form S-3 in order to allow the Company to be eligible to
file registration statements on Form S-3.

     SECTION 8.  Miscellaneous.

         (a) No Conflicting Agreements. Except for the piggyback registration
rights granted to Merck KGaA under its Development and License Agreement dated
December 14, 1998 with the Company for which the Company will use its
reasonable efforts to obtain a waiver or offer Merck KGaA a separate shelf
registration:

                                      -12-

<PAGE>

          (i) The Company is not, as of the date hereof, a party to, nor shall
     it, on or after the date of this Agreement, enter into, any agreement with
     respect to its securities that conflicts with the rights granted to the
     Holders of Registrable Securities in this Agreement.

          (ii) The Company represents and warrants that the rights granted to
     the Holders of Registrable Securities hereunder do not in any way conflict
     with the rights granted to the holders of the Company's securities under
     any other agreements.

         (b) Amendments and Waivers. The provisions of this Agreement,
including the provisions of this sentence, may not be amended, modified or
supplemented, and waivers or consents to departures from the provisions hereof
may not be given, unless the Company has obtained the written consent of
Holders of a majority of the then outstanding Underlying Common Stock
constituting Registrable Securities (with Holders of Notes deemed to be the
Holders, for purposes of this Section, of the number of outstanding shares of
Underlying Common Stock into which such Notes are or would be convertible or
exchangeable as of the date on which such consent is requested).
Notwithstanding the foregoing, a waiver or consent to depart from the
provisions hereof with respect to a matter that relates exclusively to the
rights of Holders of Registrable Securities whose securities are being sold
pursuant to a Registration Statement and that does not directly or indirectly
affect the rights of other Holders of Registrable Securities may be given by
Holders of at least a majority of the Registrable Securities being sold by such
Holders pursuant to such Registration Statement; provided, that the provisions
of this sentence may not be amended, modified, or supplemented except in
accordance with the provisions of the immediately preceding sentence. Each
Holder of Registrable Securities outstanding at the time of any such amendment,
modification, supplement, waiver or consent or thereafter shall be bound by any
such amendment, modification, supplement, waiver or consent effected pursuant
to this Section 8(b), whether or not any notice, writing or marking indicating
such amendment, modification, supplement, waiver or consent appears on the
Registrable Securities or is delivered to such Holder.

         (c) Notices. All notices and other communications provided for or
permitted hereunder shall be made in writing by hand delivery, by telecopier,
by courier guaranteeing overnight delivery or by first-class mail, return
receipt requested, and shall be deemed given (i) when made, if made by hand
delivery, (ii) upon confirmation, if made by telecopier, (iii) one (1) Business
Day after being deposited with such courier, if made by overnight courier or
(iv) on the date indicated on the notice of receipt, if made by first-class
mail, to the parties as follows:

          (w)  if to a Holder of Registrable Securities, at the most current
               address given by such Holder to the Company in a Notice and
               Questionnaire or any amendment thereto;

          (x)  if to the Company, to:

                    ImClone Systems  Incorporated
                    180 Varick Street, 7th Floor
                    New York, NY 10014
                    Attention:  General Counsel
                    Telecopy No.:  (212) 645-2054

                    and

                    Davis Polk & Wardwell
                    450 Lexington Avenue
                    New York, NY 10017
                    Attention:  Richard A. Drucker, Esq.
                    Telecopy No.: (212) 450-4800

                                      -13-

<PAGE>

          (y)  if to the Initial Purchasers, to:

                    Morgan Stanley & Co. Incorporated
                    1585 Broadway
                    New York, New York
                    Attention:  Equity Capital Markets
                    Telecopy No.: (212) 761-0538

                    and

                    Ropes & Gray
                    One International Place
                    Boston, MA  02110
                    Attention:  Patrick O'Brien, Esq.
                    Telecopy No.  (617) 951-7050

or to such other address as such person may have furnished to the other persons
identified in this Section 8(c) in writing in accordance herewith.

         (d) Approval of Holders. Whenever the consent or approval of Holders
of a specified percentage of Registrable Securities is required hereunder,
Registrable Securities held by the Company or its affiliates (as such term is
defined in Rule 405 under the Securities Act) (other than the Initial
Purchasers or subsequent Holders of Registrable Securities if such subsequent
Holders are deemed to be such affiliates solely by reason of their holdings of
such Registrable Securities) shall not be counted in determining whether such
consent or approval was given by the Holders of such required percentage.

         (e) Successors and Assigns. Any person who purchases any Registrable
Securities from the Initial Purchasers shall be deemed, for purposes of this
Agreement, to be an assignee of the Initial Purchasers. This Agreement shall
inure to the benefit of and be binding upon the successors and assigns of each
of the parties and shall inure to the benefit of and be binding upon each
Holder of any Registrable Securities.

         (f) Counterparts. This Agreement may be executed in any number of
counterparts and by the parties hereto in separate counterparts, each of which
when so executed shall be deemed to be original and all of which taken together
shall constitute one and the same agreement.

         (g) Headings. The headings in this Agreement are for convenience of
reference only and shall not limit or otherwise affect the meaning hereof.

         (h) Governing Law. THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK, WITHOUT REGARD TO CONFLICTS
OF LAWS PRINCIPLES THEREOF.

         (i) Severability. If any term provision, covenant or restriction of
this Agreement is held to be invalid, illegal, void or unenforceable, the
remainder of the terms, provisions, covenants and restrictions set forth herein
shall remain in full force and effect and shall in no way be affected, impaired
or invalidated thereby, and the parties hereto shall use their best efforts to
find and employ an alternative means to achieve the same or substantially the
same result as that contemplated by such term, provision, covenant or
restriction, it being intended that all of the rights and privileges of the
parties shall be enforceable to the fullest extent permitted by law.

         (j) Entire Agreement. This Agreement is intended by the parties as a
final expression of their agreement and is intended to be a complete and
exclusive statement of the agreement and understanding of the parties hereto in
respect of the subject matter contained herein and the registration rights
granted by the Company with respect to the Registrable Securities. Except as
provided in the Purchase Agreement, there are no restrictions, promises,
warranties or undertakings, other than those set forth or referred to herein,
with respect to the registration rights

                                      -14-

<PAGE>

granted by the Company with respect to the Registrable Securities. This
Agreement supersedes all prior agreements and undertakings among the parties
with respect to such registration rights. No party hereto shall have any
rights, duties or obligations other than those specifically set forth in this
Agreement. In no event will such methods of distribution take the form of an
underwritten offering of the Registrable Securities without the prior agreement
of the Company.

         (k) Termination. This Agreement and the obligations of the parties
hereunder shall terminate upon the end of the Effectiveness Period, except for
any liabilities or obligations under Section 4, 5 or 6 hereof and the
obligations to make payments of and provide for liquidated damages under
Section 2(e) hereof to the extent such damages accrue prior to the end of the
Effectiveness Period, each of which shall remain in effect in accordance with
its terms.

                                      -15-

<PAGE>

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

                                    IMCLONE SYSTEMS INCORPORATED

                                    By /s/ Carl S. Goldfischer
                                      ---------------------------------
                                      Name:  Carl S. Goldfischer
                                      Title: Vice President, Finance and
                                             Chief Financial Officer

Confirmed and accepted as of
the date first above written:

MORGAN STANLEY & CO. INCORPORATED
MERRILL LYNCH, PIERCE, FENNER & SMITH INCORPORATED

By: Morgan Stanley & Co. Incorporated

By  /s/ John D. Tyree
  -------------------------------
Name: John D. Tyree
Title:   Vice President

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