EXHIBIT 4.1
                                                                     -----------

                              NOTICE OF REDEMPTION

                                       OF

                                CLASS B WARRANTS


To the Holders of Outstanding
Class B Warrants of Vion Pharmaceuticals, Inc.

                  We hereby give notice that we are redeeming all of our
outstanding Class B warrants on April 27, 2000. We are exercising this right
pursuant to the terms of the warrant agreement dated as of August 14, 1995
governing our Class B warrants.

TERMS OF REDEMPTION; CESSATION OF RIGHTS

                  Redemption Date:          April 27, 2000

                  Redemption Price:         $.05 per Warrant

                  The right of our Class B warrant holders to exercise their
warrants to purchase shares of our common stock will terminate at 5:00 p.m. New
York time on April 26, 2000 (one day prior to the Redemption Date). After such
time, holders of Class B warrants will have no rights except to receive, upon
surrender of their warrants, the Redemption Price. The Redemption Price is
substantially less than either (i) the current market price of the shares of
common stock receivable upon exercise of the Class B warrants or (ii) the
current price that could be obtained upon the sale of the Class B warrants in
the open market.

REDEMPTION PROCEDURE

                  Payment of the amount to be received on redemption will be
made by us upon the presentation and surrender of the Class B warrants for
payment at any time on or after the Redemption Date. To surrender Class B
warrants for redemption, holders should deliver certificates representing their
Class B warrants to American Stock Transfer & Trust Company, our warrant agent,
at the following address:

                     American Stock Transfer & Trust Company
                           40 Wall Street - 46th Floor
                            New York, New York 10005
                            Telephone: (718) 921-8100

EXERCISE PROCEDURE (EXPIRES 5:00 P.M. NEW YORK TIME ON APRIL 26, 2000)

                  In lieu of surrendering the warrants for redemption, Class B
warrant holders may, at their option, exercise their warrants to purchase our
common stock. Each Class B warrant entitles the holder to purchase at a price of
$6.23 one share of common stock.



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                  The Class B warrants may be exercised by delivery of the Class
B warrant certificates to American Stock Transfer and Trust Company at the
address set forth above under "Redemption Procedure" accompanied by a bank or
certified check made payable to Vion Pharmaceuticals, Inc. for the full amount
of the Exercise Price ($6.23 for each warrant exercised). The subscription form
on the reverse side of each warrant must be completed in full and signed by the
warrant holder and the signature guaranteed by an eligible institution. The
method of delivery of the warrant certificates is at the option and risk of the
holder, but if mail is used, registered mail properly insured is suggested.

                  The warrant certificate and the payment of the Exercise Price
must be received by the warrant agent prior to 5:00 p.m. New York time on April
26, 2000. Warrants which are received after such date will not be exercised, but
will be redeemed. Provided that a notice of exercise and payment is received by
the warrant agent prior to 5:00 p.m. New York time on April 26, 2000,
broker-dealers shall have three business days to deliver warrant certificates to
the warrant agent.

                  ANY WARRANT RECEIVED WHICH IS NOT ACCOMPANIED BY PAYMENT OF
THE EXERCISE PRICE OR WHICH IS RECEIVED WITHOUT THE SUBSCRIPTION FORM HAVING
BEEN COMPLETED AND SIGNED WILL BE DEEMED TO HAVE BEEN DELIVERED FOR REDEMPTION,
AND NOT FOR EXERCISE.

SOLICITATION AGENT

                  Representatives of D.H. Blair Investment Banking Corp. ("D.H.
Blair") are available to assist holders in the exercise of their Class B
warrants. To receive such assistance, please contact David Nachamie at (212)
495-4105 or Martin A. Bell at (212) 495-4594. Pursuant to the terms of the
warrant agreement governing the Class B warrants, we must pay a 5% fee to D.H.
Blair for each Class B warrant that is exercised if the exercise is solicited by
D.H. Blair (a portion of this fee may be re-allowed by D.H. Blair to other
dealers who solicit the exercise).

INFORMATION AGENT

                  The Company has retained an information agent for the exercise
and/or redemption of the warrants. The information agent may be reached at:

                     Corporate Investor Communications, Inc.
                                111 Commerce Road
                           Carlstadt, New Jersey 07072
                            Telephone: 1-877-842-2409


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                                [VION LETTERHEAD]



                      IMPORTANT - IMMEDIATE ACTION REQUIRED


                                                                  March 27, 2000


Dear Class B Warrant Holder:

                  Vion Pharmaceuticals, Inc. has called for redemption of all of
its outstanding Class B warrants for cash at the redemption price of $.05 per
warrant on April 27, 2000. A copy of the Notice of Redemption, a Prospectus and
our 1998 Annual Report is enclosed.

                  We are entitled to redeem the Class B warrants because we have
satisfied the condition of redemption - namely, that the average closing price
of our common stock exceeded $9.80 for a period of 30 consecutive business days
ending within 15 days of the date of this notice. The Class B warrants are
exercisable at an exercise price of $6.23 for one share of common stock. After
5:00 p.m., New York time on April 26, 2000, the Class B warrants will no longer
be exercisable for shares of common stock and you will only have the right to
receive the redemption price.

                  We urge you to consider the following two alternatives to
redemption which are available to you and which may be more beneficial to you
than redemption:

1.       Exercise of the Class B warrants.

                  Each Class B warrant is exercisable at an exercise price of
$6.23 for one share of our common stock. The closing price of our common stock
as reported by the Nasdaq National Market ranged from a high of $29.00 to a low
of $13.88 during the period from February 11, 2000 through March 23, 2000.
Current market quotations are available from the Nasdaq National Market or your
broker.

2.       Sale of the Class B warrants in the open market.

                  The Class B warrants may be sold in the open market. The
closing price of our Class B warrants as reported by the Nasdaq SmallCap Market
ranged from a high of $22.88 to a low of $7.50 during the period from February
11, 2000 through March 23, 2000. You should consult your own broker as to the
procedure for selling your warrants and for current market quotations.



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                  The procedures for exercise and/or redemption of the warrants
are set forth in the accompanying Notice of Redemption.

                  Questions and requests for assistance should be directed to
our information agent, Corporate Investor Communications, Inc. at
1-877-842-2409.

                                                  Very truly yours,

                                                  VION PHARMACEUTICALS, INC.

                                                  /s/ Alan Kessman
                                                  -----------------------------
                                                  Alan Kessman
                                                  President and Chief Executive
                                                  Officer