Exhibit 4.3


THESE SECURITIES HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933 OR
ANY APPLICABLE STATE SECURITIES LAWS. THEY MAY NOT BE SOLD, OFFERED FOR SALE,
PLEDGED OR HYPOTHECATED OR OTHERWISE TRANSFERRED IN THE ABSENCE OF A
REGISTRATION STATEMENT IN EFFECT WITH RESPECT TO THE SECURITIES UNDER SUCH ACT
OR AN EXEMPTION THEREFROM. ANY SUCH TRANSFER MAY ALSO BE SUBJECT TO APPLICABLE
STATE SECURITIES LAWS.



                                  PROCEPT, INC.


                  Class B Warrant for the Purchase of Shares of
                  ---------------------------------------------
                                  Common Stock
                                  ------------


No. CB-1                                                        2,660,746 Shares


                  FOR VALUE RECEIVED, PROCEPT, INC., a Delaware corporation (the
"Company"), hereby certifies that the Aries Domestic Fund, LP or its registered
assigns (the "Holder") is entitled to purchase from the Company, subject to the
provisions of this Warrant (the "Warrant"), at any time commencing upon the date
hereof (the "Initial Exercise Date"), and prior to 5:00 P.M., New York City
time, on the date which is five (5) years from the date hereof (the "Termination
Date"), June 30, 2002 fully paid and non-assessable shares of the Common Stock,
$.01 par value, of the Company ("Common Stock"), at an exercise price of $0.5859
per share of Common Stock for an aggregate exercise price of One Million Five
Hundred Fifty-Eight Thousand Nine Hundred Thirty One Dollars and Eight Cents
($1,558,931.08) (the aggregate purchase price payable for the Warrant Shares
hereunder is hereinafter sometimes referred to as the "Aggregate Exercise
Price"). The number of shares of Common Stock to be received upon exercise of
this Warrant and the price to be paid for each share of Common Stock are subject
to possible adjustment from time to time as hereinafter set forth. The shares of
Common Stock or other securities or property deliverable upon such exercise as
adjusted from time to time is hereinafter sometimes referred to as the "Warrant
Shares." The exercise price of a share of Common Stock in effect at any time and
as adjusted from time to time is hereinafter sometimes referred to as the "Per
Share Exercise Price." The Per Share Exercise Price is subject to adjustment as
hereinafter provided; in the event of any such adjustment, the number of Warrant
Shares shall be adjusted by dividing the Aggregate Exercise Price by the Per
Share Exercise Price in effect immediately after such adjustment. The Aggregate
Exercise Price is not subject to adjustment.


<PAGE>


                  1.       Exercise of Warrant.

                  (a) This Warrant may be exercised in whole or in part, at any
time by the Holder commencing on the Initial Exercise Date and prior to the
Termination Date, by presentation and surrender of this Warrant, together with
the duly executed subscription form attached at the end hereof, at the address
set forth in subsection 8(a) hereof, together with payment, by certified or
official bank check or wire transfer payable to the order of the Company, of the
Aggregate Exercise Price or the proportionate part thereof if exercised in part.

                  (b) If this Warrant is exercised in part only, the Company
shall, upon presentation of this Warrant upon such exercise, execute and deliver
(along with the certificate for the Warrant Shares purchased) a new Warrant
evidencing the rights of the Holder hereof to purchase the balance of the
Warrant Shares purchasable hereunder upon the same terms and conditions as
herein set forth. Upon proper exercise of this Warrant, the Company promptly
shall deliver certificates for the Warrant Shares to the Holder duly legended as
authorized by the subscription form. No fractional shares or scrip representing
fractional shares shall be issued upon exercise of this Warrant; provided that
the Company shall pay to the holders of the Warrant cash in lieu of such
fractional shares.

                  2. Reservation of Warrant Shares; Fully Paid Shares; Taxes.
The Company hereby represents that it has, and until expiration of this Warrant
agrees that it shall, reserve for issuance or delivery upon exercise of this
Warrant, such number of shares of the Common Stock as shall be required for
issuance and/or delivery upon exercise of this Warrant in full, and agrees that
all Warrant Shares so issued and/or delivered will be validly issued, fully paid
and non-assessable, and further agrees to pay all taxes and charges that may be
imposed upon such issuance and/or delivery.

                  3.       Protection Against Dilution.

                  (a) In the event the Company shall, at any time or from time
to time after the date of issuance of this Warrant, issue or distribute to all
of the holders of its shares of Common Stock evidence of its indebtedness, any
other securities of the Company or any cash, property or other assets (any such
event being herein called a "Special Dividend"), the Per Share Exercise Price
shall be adjusted by multiplying the Per Share Exercise Price then in effect by
a fraction, the numerator of which shall be the then Current Market Price (as
defined in paragraph 3(k) below) of the Common Stock, less the Current Market
Price of the Special Dividend issued or distributed in respect of one share of
Common Stock, and the denominator of which shall be the Current Market Price of
the Common Stock. Such adjustment shall be made successively whenever such a
record date is fixed. Such adjustment shall be made whenever any such
distribution is made and shall become effective immediately after the record
date for the determination of shareholders entitled to receive such distribution
unless such distribution is not ultimately made.

                  (b) In case the Company shall hereafter (i) pay a dividend or
make a distribution on its capital stock in shares of Common Stock, (ii)
subdivide its outstanding shares of Common


                                        2

<PAGE>


Stock into a greater number of shares, (iii) combine its outstanding shares of
Common Stock into a smaller number of shares or (iv) issue by reclassification
of its Common Stock any shares of capital stock of the Company, the Per Share
Exercise Price shall be adjusted to be equal to a fraction, the numerator of
which shall be the Aggregate Exercise Price and the denominator of which shall
be the number of shares of Common Stock or other capital stock of the Company
issuable upon exercise of this Warrant assuming this Warrant had been exercised
immediately prior to such action. An adjustment made pursuant to this subsection
3(b) shall become effective immediately after the record date in the case of a
dividend or distribution and shall become effective immediately after the
effective date in the case of a subdivision, combination or reclassification.

                  (c)(i) Except as provided in subsections 3(a) and 3(b)(i), in
the event the Company shall hereafter issue or sell any Common Stock, any
securities convertible into Common Stock or any rights, options or warrants to
purchase Common Stock or securities convertible into Common Stock, in each case
for a price per share or entitling the holders thereof to purchase Common Stock
at a price per share (determined by dividing (i) the total amount, if any,
received or receivable by the Company in consideration of the issuance or sale
of such securities plus the consideration, if any, payable to the Company upon
exercise or conversion thereof (collectively, the "Total Consideration") by (ii)
the number of additional shares of Common Stock issued, sold or issuable upon
exercise or conversion of such securities) which is less than the then Current
Market Price of the Common Stock (as defined below) but not below the current
Per Share Exercise Price (which event is governed by subsection 3(c)(ii)), the
Per Share Exercise Price shall be adjusted as of the date of such issuance or
sale by multiplying the Per Share Exercise Price then in effect by a fraction,
the numerator of which shall be (x) the sum of (A) the number of shares of
Common Stock outstanding on the record date of such issuance or sale plus (B)
the Total Consideration divided by the Current Market Price of the Common Stock,
and the denominator of which shall be (y) the number of shares of Common Stock
outstanding on the record date of such issuance or sale plus the maximum number
of additional shares of Common Stock issued, sold or issuable upon exercise or
conversion of such securities.

                  (ii) Except as provided in subsection 3(a) and 3(b)(i), in the
event the Company shall hereafter issue or sell any Common Stock, any securities
convertible into Common Stock or any rights, options or warrants to purchase
Common Stock or securities convertible into Common Stock, in each case for a
price per share or entitling the holders thereof to purchase Common Stock at a
price per share (the "Issue Price"), (determined by dividing (i) the Total
Consideration by (ii) the number of additional shares of Common Stock issuable
upon exercise or conversion of such securities) which is less than the then
current Per Share Exercise Price in effect on the record date of such issuance,
the Per Share Exercise Price shall be adjusted to equal the Issue Price.

                  (d) In the event of any capital reorganization or
reclassification, or any consolidation or merger to which the Company is a party
other than a merger or consolidation in which the Company is the continuing
corporation, or in case of any sale or conveyance to another entity of the
property of the Company as an entirety or substantially as an entirety, or in
the case of any statutory exchange of securities with another corporation
(including any exchange effected


                                        3

<PAGE>


in connection with a merger of a third corporation into the Company), the Holder
of this Warrant shall have the right thereafter to receive on the exercise of
this Warrant the kind and amount of securities, cash or other property which the
Holder would have owned or have been entitled to receive immediately after such
reorganization, reclassification, consolidation, merger, statutory exchange,
sale or conveyance had this Warrant been exercised immediately prior to the
effective date of such reorganization, reclassification, consolidation, merger,
statutory exchange, sale or conveyance and in any such case, if necessary,
appropriate adjustment shall be made in the application of the provisions set
forth in this Section 3 with respect to the rights and interests thereafter of
the Holder of this Warrant to the end that the provisions set forth in this
Section 3 shall thereafter correspondingly be made applicable, as nearly as may
reasonably be, in relation to any shares of stock or other securities or
property thereafter deliverable on the exercise of this Warrant. The above
provisions of this subsection 3(e) shall similarly apply to successive
reorganizations, reclassifications, consolidations, mergers, statutory
exchanges, sales or conveyances. The issuer of any shares of stock or other
securities or property thereafter deliverable on the exercise of this Warrant
shall be responsible for all of the agreements and obligations of the Company
hereunder. Notice of any such reorganization, reclassification, consolidation,
merger, statutory exchange, sale or conveyance and of said provisions so
proposed to be made, shall be mailed to the Holders of the Warrants not less
than 30 days prior to such event. A sale of all or substantially all of the
assets of the Company for a consideration consisting primarily of securities
shall be deemed a consolidation or merger for the foregoing purposes.

                  (e) In case any event shall occur as to which the other
provisions of this Section 3 are not strictly applicable but as to which the
failure to make any adjustment would not fairly protect the purchase rights
represented by this Warrant in accordance with the essential intent and
principles hereof then, in each such case, the Holders of Warrants representing
the right to purchase a majority of the Warrant Shares subject to all
outstanding Warrants may appoint a firm of independent public accountants of
recognized national standing reasonably acceptable to the Company, which shall
give their opinion as to the adjustment, if any, on a basis consistent with the
essential intent and principles established herein, necessary to preserve the
purchase rights represented by the Warrants. Upon receipt of such opinion, the
Company will promptly mail a copy thereof to the Holder of this Warrant and
shall make the adjustments described therein. The fees and expenses of such
independent public accountants shall be borne by the Company.

                  (f) Whenever the Per Share Exercise Price payable upon
exercise of each Warrant is adjusted pursuant to this Section 3, the number of
shares of Common Stock underlying a Warrant shall simultaneously be adjusted to
equal the number obtained by dividing the Aggregate Exercise Price by the
adjusted Per Share Exercise Price.

                  (g) No adjustment in the Per Share Exercise Price shall be
required unless such adjustment would require an increase or decrease of at
least $0.01 per share of Common Stock; provided, however, that any adjustments
which by reason of this subsection 3(g) are not required to be made shall be
carried forward and taken into account in any subsequent adjustment. All
calculations under this Section 3 shall be made to the nearest cent or to the
nearest 1/100th of a share, as the case may be. Anything in this Section 3 to
the contrary notwithstanding, the


                                        4

<PAGE>


Company shall be entitled to make such reductions in the Per Share Exercise
Price, in addition to those required by this Section 3, as it in its discretion
shall deem to be advisable in order that any stock dividend, subdivision of
shares or distribution of rights to purchase stock or securities convertible or
exchangeable for stock hereafter made by the Company to its stockholders shall
not be taxable.

                  (h) Whenever the Per Share Exercise Price is adjusted as
provided in this Section 3 and upon any modification of the rights of a Holder
of Warrants in accordance with this Section 3, the Company shall promptly
obtain, at its expense, a certificate of a firm of independent public
accountants of recognized standing selected by the Board of Directors (who may
be the regular auditors of the Company) setting forth the Per Share Exercise
Price and the number of Warrant Shares after such adjustment or the effect of
such modification, a brief statement of the facts requiring such adjustment or
modification and the manner of computing the same and cause copies of such
certificate to be mailed to the Holders of the Warrants.

                  (i) If the Board of Directors of the Company shall declare any
dividend or other distribution with respect to the Common Stock, the Company
shall mail notice thereof to the Holders of the Warrants not less than 30 days
prior to the record date fixed for determining stock holders entitled to
participate in such dividend or other distribution.

                  (j) If, as a result of an adjustment made pursuant to this
Section 3, the Holder of any Warrant thereafter surrendered for exercise shall
become entitled to receive shares of two or more classes of capital stock or
shares of Common Stock and other capital stock of the Company, the Board of
Directors (whose determination shall be conclusive and shall be described in a
written notice to the Holder of any Warrant promptly after such adjustment)
shall determine the allocation of the adjusted Per Share Exercise Price between
or among shares or such classes of capital stock or shares of Common Stock and
other capital stock.

                  (k) For the purpose of any computation under Section 3 above,
the then Current Market Price per share (the "Current Market Price") shall be
deemed to be the last sale price of the Common Stock on the trading day prior to
such date or, in case no such reported sales take place on such day, the average
of the last reported bid and asked prices of the Common Stock on such day, in
either case on the principal national securities exchange on which the Common
Stock is admitted to trading or listed, or if not listed or admitted to trading
on any such exchange, the representative closing bid price of the Common Stock
as reported by the National Association of Securities Dealers, Inc. Automated
Quotations System ("NASDAQ"), or other similar organization if NASDAQ is no
longer reporting such information, or, if the Common Stock is not reported on
NASDAQ, the high per share bid price for the Common Stock in the
over-the-counter market as reported by the National Quotation Bureau or similar
organization, or if not so available, the fair market value of the Common Stock
as determined by agreement between the Company's Board of Directors, on the one
part, and the Holders of Warrants representing the right to purchase a majority
of the Warrant Shares subject to all outstanding Warrants, on the second part.
If the Board of Directors and such Holders fail to agree on the Current Market
Price within 60 days of the date of the action giving rise to any adjustment
pursuant to this Section 3, such Holders shall


                                        5

<PAGE>


be entitled to appoint a firm of independent public accountants or appraisers of
recognized national standing reasonably acceptable to the Company, which shall
give their opinion as to such Current Market Price on a basis consistent with
the essential intent and principles established herein. Upon receipt of such
opinion, the Company will promptly mail a copy thereof to the Holder of this
Warrant and shall make the adjustments described therein. The fees and expenses
of such independent public accountants or appraisers shall be borne by the
Company.

                  4. Registration Under Securities Act of 1933 . The resale of
the Warrant Shares shall be registered on the Shelf Registration Statement (as
defined in Article 8 of the Securities Purchase Agreement (the "Purchase
Agreement") dated as of June 30, 1997, by and among the Company, The Aries Fund,
a Cayman Island Trust, and The Aries Domestic Fund, L.P., a Delaware limited
partnership) and certain purchasers and the Holder of this Warrant shall have
the registration rights as provided in Article 8 of the Purchase Agreement. If
the Holder is not a party to the Purchase Agreement, by acceptance of this
Warrant the Holder agrees to comply with provisions of Article 8 of the Purchase
Agreement to the same extent as if it were a party thereto.

                  5. Limited Transferability. This Warrant may not be sold,
transferred, assigned or hypothecated by the Holder except in compliance with
the provisions of the Securities Act of 1933 (the "Act") and the applicable
state securities "blue sky" laws, and is so transferable only upon the books of
the Company which it shall cause to be maintained for such purpose. The Company
may treat the registered Holder of this Warrant as he or it appears on the
Company's books at any time as the Holder for all purposes. The Company shall
permit any Holder of a Warrant or his duly authorized attorney, upon written
request during ordinary business hours, to inspect and copy or make extracts
from its books showing the registered holders of Warrants. All Warrants issued
upon the transfer or assignment of this Warrant will be dated the same date as
this Warrant, and all rights of the holder thereof shall be identical to those
of the Holder.

                  6. Loss, etc., of Warrant. Upon receipt of evidence
satisfactory to the Company of the loss, theft, destruction or mutilation of
this Warrant, and of indemnity reasonably satisfactory to the Company, if lost,
stolen or destroyed, and upon surrender and cancellation of this Warrant, if
mutilated, the Company shall execute and deliver to the Holder a new Warrant of
like date, tenor and denomination.

                  7. Status of Holder. This Warrant does not confer upon the
Holder any right to vote or to consent to or receive notice as a stockholder of
the Company, as such, in respect of any matters whatsoever, or any other rights
or liabilities as a stockholder, prior to the exercise hereof.

                  8. Notices. No notice or other communication under this
Warrant shall be effective unless, but any notice or other communication shall
be effective and shall be deemed to have been given if, the same is in writing
and is mailed by first-class mail, postage prepaid, addressed to:


                                        6

<PAGE>


                  (a) the Company at 840 Memorial Drive, Cambridge, 
         Massachusetts, Attention: Stanley C. Erck, or such other address as
         the Company has designated in writing to the Holder; or

                  (b) the Holder at the address indicated in the notice
         provisions to the Purchase Agreement, or other such address as the
         Holder has designated in writing to the Company.

                  9. Optional Conversion. This Warrant shall be converted (the
"New Warrant Conversion") into a New Warrant (as hereinafter defined) with an
adjusted exercise price as set forth in this Section 9. "New Warrants" shall
mean a new class of warrants entitling the holders thereof to purchase, at any
time on or before the date which is five (5) years from the date hereof, one
share of Common Stock at an exercise price, subject to adjustment, equal to the
lesser of (a) $.29 and (b) fifty percent (50%) of the average closing bid price
of the Common Stock for either (i) the thirty (30) consecutive trading days
preceding the date of the Required Shareholder Approval (the "Approval Date"),
if any, or September 30, 1997, (ii) the five (5) consecutive trading days
preceding the Approval Date, if any, or September 30, 1997 or (iii) the five (5)
consecutive trading days immediately succeeding the Approval Date, if any, or
September 30, 1997 whichever is lowest. Other than the per share exercise
prices, the New Warrants shall have the same terms as the Class A and B Warrants
held by such Holder respectively (including, without limitation, an Aggregate
Exercise Price equal to the Aggregate Exercise Price of the sum of the Class A
and B Warrants Aggregate Exercise Prices prior to the New Warrant Conversion)
and all adjustments required upon an adjustment to the exercise price of the New
Warrants pursuant to the terms thereof shall be made in connection with the New
Warrant Conversion. To the extent that there is no Required Shareholder Approval
(as defined in the Purchase Agreement) necessary, the Initial Warrants shall be
converted into New Warrants on September 30, 1997, with an adjusted exercise
price equal to the lesser of (a) $.29 and (b) fifty percent (50%) of the average
closing bid price of the Common Stock for either (i) the thirty (30) consecutive
trading days preceding September 30, 1997, (ii) the five (5) consecutive trading
days preceding September 30, 1997 or (iii) the five (5) consecutive trading days
immediately succeeding September 30, 1997, whichever is lowest.

                  Notwithstanding the foregoing, the New Warrants' per share
exercise price shall be adjusted at the time of the Series B Final Closing Date
or the closing of the Company's next Qualified Offering (as these terms are
defined in the Letter of Intent between the Company and Paramount Capital Inc.,
dated June 30, 1997) if (i) the per share exercise price of the Offering
Warrants (as defined below) or (ii) the quotient of (a) the price per unit sold
in the Series B Offering or other Qualified Offering divided by (b) the quantity
of Common Stock (or any securities other than Common Stock viewed on a Common
Stock equivalent basis, collectively, the "Other Securities") included in each
unit sold in such Series B Offering or other Qualified Offering, is less than
twice the per share exercise price of the New Warrants. In such event the New
Warrants per share exercise price shall be reduced to equal 50% of the then
current per share exercise price of the Offering Warrants (as hereafter defined)
or per share exercise or offering price of the Other Securities (viewed on a
Common Stock equivalent basis). "Offering Warrants"


                                        7

<PAGE>


shall mean the warrants described in paragraph 8 of the Letter of Intent between
the Company the Holders, and Paramount Capital, Inc. dated June 30, 1997.

                  10. Headings. The headings of this Warrant have been inserted
as a matter of convenience and shall not affect the construction hereof.

                  11. Applicable Law. This Warrant shall be governed by and
construed in accordance with the law of the State of New York without giving
effect to principles of conflicts of law thereof.

                  IN WITNESS WHEREOF, the Company has caused this Warrant to be
signed by its _______________________ and its corporate seal to be hereunto
affixed and attested by its Secretary this June 30, 1997.



                                     PROCEPT, INC.



                                     By:    /s/ Stanley C. Erck
                                            -------------------------------
                                            Name:   Stanley C. Erck
                                            Title:  President


ATTEST: /s/ Lynnette Fallon



--------------------
     Secretary



[Corporate Seal]



                                        8

<PAGE>


                                  SUBSCRIPTION

                  The undersigned, ____________________________, pursuant to the
provisions of the foregoing Warrant, hereby elects to exercise the within
Warrant to the extent of purchasing _____________________ shares of Common Stock
thereunder and hereby makes payment of $_______________ by certified or official
bank check in payment of the per share exercise price therefor.

Dated:_______________                    Signature:_____________________________

                                         Address:_______________________________


                                   ASSIGNMENT

                  FOR VALUE RECEIVED _______________________________________
hereby sells, assigns and transfers unto _____________________________________
the foregoing Warrant and all rights evidenced thereby, and does irrevocably
constitute and appoint _____________________________, attorney, to transfer said
Warrant on the books of Procept, Inc.


Dated:_______________                    Signature:_____________________________

                                          Address:______________________________


                               PARTIAL ASSIGNMENT

                  FOR VALUE RECEIVED __________________________ hereby assigns
and transfers unto _________________________ the right to purchase __________
shares of the Common Stock, no par value per share, of Procept Inc. covered by
the foregoing Warrant, and a proportionate part of said Warrant and the rights
evidenced thereby, and does irrevocably constitute and appoint
__________________________, attorney, to transfer that part of said Warrant on
the books of Procept, Inc.


Dated:_______________                      Signature:___________________________

                                           Address:_____________________________



                                        9