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                                                                     EXHIBIT 4.1

                           VION PHARMACEUTICALS, INC.
                        2000 EMPLOYEE STOCK PURCHASE PLAN


         1. Purpose. The purpose of the Vion Pharmaceuticals, Inc. 2000
Employee Stock Purchase Plan is to provide Eligible Employees of the Company and
its Subsidiaries with a means to acquire shares of Common Stock through payroll
deductions. The Plan is intended to qualify as an "employee stock purchase plan"
under Section 423 of the Code and will be interpreted and construed accordingly.

         2. Definitions. For purposes of the Plan, the following terms have the
following meanings:

                  (a) "ACCOUNT" shall mean the bookkeeping account established
in the name of each Participant to reflect the payroll deductions made on behalf
of the Participant.

                  (b) "BOARD" shall mean the Board of Directors of the Company.

                  (c) "CODE" shall mean the Internal Revenue Code of 1986, as
amended.

                  (d) "COMMITTEE" shall mean the committee appointed by the
Board to administer the Plan pursuant to Section 4.

                  (e) "COMMON STOCK" shall mean the Company's common stock,
$0.01 par value.

                  (f) "COMPANY" shall mean Vion Pharmaceuticals, Inc., a
Delaware corporation, and any successor thereto.

                  (g) "COMPENSATION" shall mean the base compensation paid by
the Company or a Subsidiary to a Participant which is required to be reported as
wages on the Participant's Form W-2, including such additional amounts which are
not includable in gross income by reason of Sections 125, 402(e)(3) or
402(h)(1)(B) of the Code, and excluding any bonuses, overtime pay, severance,
sick leave, expense and relocation allowances, gains from the exercise of stock
options and other irregular payments (except commissions).

                  (h) "ELIGIBLE EMPLOYEE" shall mean, with respect to an
Offering Period, any individual, who is an employee of the Company or a
Subsidiary on the Enrollment Date and satisfies each of the following criteria:

                           (1) the employee does not immediately after the
grant, directly or indirectly, own stock (as defined by the Code) and/or hold
outstanding options to purchase stock possessing five percent (5%) or more of
the total combined voting power or value of all classes of stock of the Company
or a "parent corporation" or Subsidiary of the Company;











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                           (2) the employee has been employed by the Company or
its Subsidiaries for at least six (6) months, provided that the Committee may,
on a uniform and nondiscriminatory basis, decrease or increase (but not in
excess of two (2) years) such minimum employment period for future Offering
Periods;

                           (3) the employee's customary employment is for twenty
(20) hours or more per week, provided that the Committee may, on a uniform and
nondiscriminatory basis, decrease this minimum hours requirement for future
Offering Periods; and

                           (4) if specified by the Committee for a future
Offering Period, the employee customarily works a minimum of five (5) months per
year or any lesser number of months established by the Committee.

If the Committee permits any Eligible Employee of a Subsidiary to participate in
the Plan during an Offering Period, then all Eligible Employees of that
Subsidiary shall also be permitted to participate in the Plan during such
Offering Period.

                  (i) "ENROLLMENT DATE" shall mean the first day of an Offering
Period.

                  (j) "EXERCISE DATE" shall mean the last trading day of an
Offering Period.

                  (k) "FAIR MARKET VALUE" shall mean, as of any date, the
closing sale price per share of Common Stock as published by the principal
national securities exchange on which the Common Stock is traded on such date
or, if there is no sale of Common Stock on such date, the average of the bid and
asked prices on such exchange at the close of trading on such date, or if shares
of the Common Stock are not listed on a national securities exchange on such
date, the last reported trading price or, if none, the average of the bid and
asked prices in the over-the-counter market at the close of trading on such
date, or if the Common Stock is not traded on a national securities exchange or
the over-the-counter market, the value of a share of the Common Stock on such
date as determined in good-faith by the Committee.

                  (l) "OFFERING PERIOD" shall mean the six month period
commencing on January 1, 2001 or such later date as established by the
Committee, and each successive six months thereafter. To the extent permitted
under Section 423 of the Code, the Committee shall have the power to change the
duration of future Offering Periods and the commencement dates thereof.

                  (m) "PARTICIPANT" shall mean any Eligible Employee for whom an
Account is maintained under the Plan.

                  (n) "PURCHASE PRICE" shall mean, subject to adjustment
pursuant to Section 10 hereof, eighty-five percent (85%) of the lesser of the
Fair Market Value of a share of Common Stock on (i) the Enrollment Date or (ii)
the Exercise Date.







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                  (o) "PLAN" shall mean this Vion Pharmaceuticals, Inc. 2000
Employee Stock Purchase Plan, as amended from time to time.

                  (p) "SUBSIDIARY" shall mean any "subsidiary corporation" of
the Company (within the meaning of Section 424(f) of the Code) that is
designated by the Committee as an eligible Subsidiary under the Plan from time
to time.

         3. Available Shares. Subject to adjustment as provided in Section 10,
the maximum number of shares of Common Stock that may be issued under the Plan
shall not exceed 450,000 shares. Such shares may be either authorized and
unissued or held by the Company in its treasury. The Committee may cause the
Company to purchase previously issued and outstanding shares of Common Stock in
order to enable the Company to satisfy its obligations hereunder. Subject to
adjustment pursuant to Section 10 or as otherwise determined by the Board prior
to the commencement of any Offering Period, the maximum number of shares of
Common Stock a Participant may purchase during any Offering Period shall not
exceed 2,000 shares.

         4. Administration. The Plan shall be administered by the Company's
Compensation Committee or such other committee appointed by the Board from time
to time. Subject to the provisions of the Plan, the Committee, acting in its
sole and absolute discretion, shall have full power and authority to interpret
the provisions of the Plan, to change the time covered by an Offering Period, to
supervise the administration of the Plan, and to take such other action as may
be necessary or desirable in order to carry out the provisions of the Plan. A
majority of the members of the Committee shall constitute a quorum. The
Committee may act by the vote of a majority of its members present at a meeting
at which there is a quorum or by written consent. The decision of the Committee,
including questions of construction, interpretation and administration, shall be
final, binding and conclusive on all persons. The Company shall indemnify and
hold harmless each member of the Committee and any employee or director of the
Company or of a Subsidiary who provides assistance or performs services in
connection with the administration or interpretation of the Plan against any
loss, cost, liability (including any sum paid in settlement of a claim with the
approval of the Board), damage and expense (including legal and other expenses
incident thereto) arising out of or incurred in connection with the Plan, unless
and except to the extent attributable to such person's fraud or willful
misconduct.

         5. Eligibility and Enrollment. An Eligible Employee shall become a
Participant on the first Enrollment Date occurring on or after his or her
satisfaction of the Plan's eligibility requirements and completion of a Plan
enrollment form authorizing payroll deductions. Such enrollment form must be
filed with the Company prior to the applicable Offering Period and during the
enrollment period established by the Company. Payroll deductions for a
Participant shall commence with the first payroll and shall end with the last
payroll in the Offering Period to which such authorization is applicable, unless
sooner terminated by the Participant in accordance with the provisions hereof.
Notwithstanding any provisions of the Plan to the contrary, to the extent
required by applicable law, no Participant may be granted the right to purchase
Common Stock under the Plan if and to the extent that the Participant's right to
purchase stock under all employee stock purchase plans (within the meaning of
Section 423 of the Code) of the Company or a Subsidiary would accrue at a rate
which exceeds $25,000 in Fair Market Value (determined











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at the time of grant) in accordance with the Code for each calendar year in
which such right is outstanding.

         6. Payroll Deduction. At the time a Participant enrolls in the Plan, he
or she must elect the amount to be deducted from each paycheck during the
Offering Period(s) covered by the election, provided that (i) unless otherwise
determined by the Committee with respect to future Offering Periods, no more
than TEN PERCENT (10%) of a Participant's Compensation may be withheld under the
Plan on any pay date, and (ii) the Committee, acting in its discretion and in a
uniform and nondiscriminatory manner, may establish a minimum required amount or
percentage of Compensation which must be withheld during an Offering Period. All
payroll deductions made for a Participant shall be credited to the Participant's
Account. Interest shall not accrue on any amounts credited to a Participant's
Account. The rate of a Participant's contribution, once established, shall
remain in effect for all subsequent Offering Periods unless changed by the
Participant in writing at such time and in such manner as the Committee may
prescribe.

         7. Purchase of Shares. On each Exercise Date, the amount credited to a
Participant's Account shall be used to purchase a number of shares of Common
Stock, the number of which will be determined by dividing the amount credited to
the Participant's Account by the Purchase Price per share. Any amount remaining
in the Participant's Account shall be credited to the Participant's Account as
of the beginning of the next Offering Period. If the total number of shares of
Common Stock to be purchased as of an Exercise Date, when aggregated with shares
of Common Stock previously-purchased under the Plan, exceeds the number of
shares then authorized under the Plan, a pro-rata allocation of the available
shares shall be made among the Participants based upon the amounts in their
respective Accounts as of the Exercise Date.

         8. Discontinuance or Withdrawal; Withholding Changes.

                  (a) Discontinuance or Withdrawal. At any time during an
Offering Period, a Participant may notify the Company that he or she wishes to
discontinue contributions under the Plan. This notice shall be in writing and
shall become effective as soon as practicable following its receipt by the
Company. A Participant may elect to withdraw all, but not less than all, of the
amount of his or her Account at any time during an Offering Period, except on
the Exercise Date with respect to that Offering Period. If a withdrawal is made
during an Offering Period, no further contributions shall be permitted during
that Offering Period by the withdrawing Participant.

                  (b) Withholding Changes. At any time during an Offering
Period, a Participant may increase or decrease the rate of his or her payroll
deductions by completing or filing with the Company a new enrollment form
authorizing a change in payroll deduction rate. The Committee may, in its
discretion, limit the number of payroll deduction rate changes during any
Offering Period. The change in rate shall be effective as soon as practicable
following the Company's receipt of the new enrollment form.

                  (c) Termination of Employment. Any Participant whose
employment with the Company and its Subsidiaries is terminated for any reason
before an Exercise Date shall









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thereupon cease being a Participant. The total amount credited to the
Participant's Account during the Offering Period shall be returned to the
Participant or, in the case of a deceased Participant, to the Participant's
beneficiary, as soon as practicable after the Participant's termination of
employment.

         9. Rights as a Stockholder. No shares of Common Stock shall be issued
under the Plan until full payment therefor has been made. A Participant shall
have no rights as a stockholder with respect to any shares until the date a
stock certificate for such shares is issued to him or her. Except as otherwise
specifically provided herein, no adjustments shall be made for dividends or
distributions of other rights for which the record date is prior to the date
such stock certificate is issued.

         10. Capital Changes; Reorganization; Sale.

                  (a) Adjustments Upon Changes in Capitalization. The aggregate
number and class of shares of Common Stock which may be issued under the Plan
and purchased by a Participant, as well as the number and class of shares of
Common Stock and the price per share covered by each right outstanding under the
Plan which has not yet been exercised, shall be adjusted proportionately or as
otherwise appropriate to reflect any increase or decrease in the number of
issued shares of Common Stock resulting from a split-up or consolidation of
shares or any like capital adjustment, or the payment of a stock dividend,
and/or to reflect a change in the character or class of shares covered by the
Plan arising from a readjustment or recapitalization.

                  (b) Cash, Stock or Other Property for Stock. Except as
otherwise provided in this Section, in the event of an Exchange Transaction (as
defined below), each Participant shall be permitted to purchase Common Stock
with the balance of his or her Account immediately prior to such Exchange
Transaction, and any amount credited to a Participant's Account which is not
used to purchase Common Stock before the Exchange Transaction shall be
distributed to the Participant. Notwithstanding the preceding sentence, (i) if,
as part of the Exchange Transaction, the stockholders of the Company receive
capital stock of another corporation ("Exchange Stock") in exchange for their
shares of Common Stock (whether or not such Exchange Stock is the sole
consideration), and if the Board, in its sole discretion, so directs, then the
rights of all Participants to purchase shares of Common Stock shall be converted
into rights to purchase shares of Exchange Stock on an economically equivalent
basis; and (ii) the Board, acting in its discretion, may suspend operation of
the Plan as of any date that occurs after a contract is made which, if
consummated, would result in an Exchange Transaction and before the Exchange
Transaction is consummated.

                  (c) Definition of Exchange Transaction. For purposes hereof,
the term "Exchange Transaction" shall mean a merger (other than a merger of the
Company in which the holders of Common Stock immediately prior to the merger
have the same proportionate ownership of Common Stock in the surviving
corporation immediately after the merger), consolidation, acquisition of
property or stock, separation, reorganization (other than a mere reincorporation
or the creation of a holding company), liquidation of the Company or any other
similar transaction or event so designated by the Board in its sole discretion,
as a result of which









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the stockholders of the Company receive cash, stock or other property in
exchange for or in connection with their shares of Common Stock.

                  (d) Determination of Board to be Final. All adjustments under
this Section 10 shall be made by the Board, and its determination as to what
adjustments shall be made, and the extent thereof, shall be final, binding and
conclusive.

         11. Amendment and Termination. The Board may amend or terminate the
Plan at any time; provided, however, that, except as otherwise provided in
Section 10 hereof, any amendment which increases the aggregate number of shares
of Common Stock which may be issued under the Plan, modifies the corporations or
class of corporations whose employees are offered options under the Plan or
otherwise requires stockholder approval under applicable law shall be subject to
the approval of the Company's stockholders.

         12. Transferability. The rights of a Participant to purchase Common
Stock under the Plan are not assignable or transferable and may only be
exercised during the Participant's lifetime by the Participant. A Participant
may file a written designation of a beneficiary who is to receive the amount
credited to the Participant's Account in the event of the Participant's death
during an Offering Period. A Participant's beneficiary designation may be
changed by the Participant at any time by written notice. In the event of the
death of a Participant and in the absence of a validly designated beneficiary
who is living at the time of the Participant's death, the Participant's estate
shall be deemed to be his or her designated beneficiary.

         13. No Rights Conferred. Nothing contained in the Plan shall be deemed
to give any individual any right to be retained in the employ of the Company and
its Subsidiaries or to interfere with the right of the Company and its
Subsidiaries to discharge him or her at any time.

         14. Use of Funds. All payroll deductions received or held by the
Company under the Plan may be used by the Company for any corporate purpose, and
the Company shall not be obligated to segregate such payroll deductions.

         15. Legal Requirements. The Committee may impose such other conditions
with respect to the purchase of Common Stock hereunder, including, without
limitation, any conditions relating to the application of federal or state
securities laws and/or any exchange or listing requirements as it may deem
necessary or advisable.

         16. Governing Law. The Plan shall be governed by the laws of the State
of New York, without regard to its principles of conflicts of law.

         17. Decisions and Determinations. Any decision or determination made by
the Board pursuant to the provisions hereof and, except to the extent rights or
powers under the Plan are reserved specifically to the discretion of the Board,
all decisions and determinations of the Committee are final, binding and
conclusive.








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         18. Effective Date. The Plan shall be effective on March 2, 2000, the
date on which the Plan was adopted by the Board, subject to approval by the
Company's stockholders within twelve (12) months thereafter.