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                                                                     EXHIBIT 4.3

                           [Form of Right Certificate]

Certificate No. [R]-
                        Rights

               NOT EXERCISABLE AFTER APRIL 19, 2012, OR EARLIER IF REDEEMED BY
               THE COMPANY. THE RIGHTS ARE SUBJECT TO REDEMPTION, AT THE OPTION
               OF THE COMPANY, AT $.001 PER RIGHT, ON THE TERMS SET FORTH IN THE
               RIGHTS AGREEMENT. RIGHTS BENEFICIALLY OWNED BY AN ACQUIRING
               PERSON OR AN AFFILIATE OR ASSOCIATE OF AN ACQUIRING PERSON (AS
               SUCH TERMS ARE DEFINED IN THE RIGHTS AGREEMENT) AND BY ANY
               SUBSEQUENT HOLDER OF SUCH RIGHTS ARE NULL AND VOID AND
               NONTRANSFERABLE.

                                Right Certificate

                              CENTRA SOFTWARE, INC.


         This certifies that,                           or registered assigns,
is the registered owner of the number of Rights set forth above, each of which
entitles the owner, thereof, subject to the terms, provisions and conditions of
the Rights Agreement dated as of April 19, 2002 (the "Rights Agreement"),
between Centra Software, Inc., a Delaware corporation (the "Company"), and
American Stock Transfer & Trust Company, a transfer agent, as Rights Agent (the
"Rights Agent"), unless the Rights evidenced hereby shall have been previously
redeemed by the Company, to purchase from the Company at any time after the
Distribution Date (as defined in the Rights Agreement) and prior to 5:00 p.m.,
Boston time, on the 10th anniversary of the date of the Rights Agreement (the
"Expiration Date"), at the office of the Rights Agent, or its successors as
Rights Agent, in Brooklyn, New York, one one-thousandth (1/1,000th) of a fully
paid, nonassessable share of Series A Participating Cumulative Preferred Stock,
par value $.001 per share, of the Company (the "Preferred Shares"), at a
purchase price per one one-thousandth (1/1,000th) of a share equal to $29 (the
"Purchase Price") payable in cash, upon presentation and surrender of this Right
Certificate with the Form of Election to Purchase duly executed.

         The Purchase Price and the number and kind of shares which may be
purchased upon exercise of each Right evidenced by this Right Certificate, as
set forth above, are the Purchase Price and the number and kind of shares which
may be so purchased as of April 19, 2002. As provided in the Rights Agreement,
the Purchase Price and the number and kind of shares which may be purchased upon
the exercise of each Right evidenced by this Right Certificate are subject to
modification and adjustment upon the happening of certain events.

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         If the Rights evidenced by this Right Certificate are at any time
beneficially owned by an Acquiring Person or an Affiliate or Associate of an
Acquiring Person (as such terms are defined in the Rights Agreement), such
Rights shall be null and void and nontransferable and the holder of any such
Right (including any purported transferee or subsequent holder) shall not have
any right to exercise or transfer any such Right.

         This Right Certificate is subject to all the terms, provisions and
conditions of the Rights Agreement, which terms, provisions and conditions are
hereby incorporated herein by reference and made a part hereof and to which
reference to the Rights Agreement is hereby made for a full description of the
rights, limitations of rights, obligations, duties and immunities hereunder of
the Rights Agent, the Company and the holders of the Right Certificates. Copies
of the Rights Agreement are on file at the above-mentioned office of the Rights
Agent and are also available from the Company upon written request.

         This Right Certificate, with or without other Right Certificates, upon
surrender at the stock transfer or corporate trust office of the Rights Agent,
may be exchanged for another Right Certificate or Right Certificates of like
tenor and date evidencing Rights entitling the holder to purchase a like
aggregate number and kind of shares as the Rights evidenced by the Right
Certificate or Right Certificates surrendered shall entitle such holder to
purchase. If this Right Certificate shall be exercised in part, the holder shall
be entitled to receive upon surrender hereof another Right Certificate or Right
Certificates for the number of whole Rights not exercised.

         Subject to the provisions of the Rights Agreement, the Rights evidenced
by this Right Certificate may be redeemed by the Company at its option at a
redemption price (in cash or shares of Common Stock or other securities of the
Company deemed by the Board of Directors to be at least equivalent in value) of
$.001 per Right (which amount shall be subject to adjustment as provided in the
Rights Agreement) at any time prior to the earlier of (i) such time as a Person
becomes an Acquiring Person and (ii) the Expiration Date.

         The Company may, but shall not be required to, issue fractions of
Preferred Shares or distribute certificates which evidence fractions of
Preferred Shares upon the exercise of any Right or Rights evidenced hereby. In
lieu of issuing fractional shares, the Company may elect to make a cash payment
as provided in the Rights Agreement for fractions of a share other than one
one-thousandth (1/1,000th) of a share or any integral multiple thereof or to
issue certificates or utilize a depository arrangement as provided in the terms
of the Rights Agreement and the Preferred Shares.

         No holder of this Right Certificate shall be entitled to vote or
receive dividends or be deemed for any purpose the holder of the Preferred
Shares or of any other securities of the Company which may at any time be
issuable on the exercise hereof, nor shall anything contained in the Rights
Agreement or herein be construed to confer upon the holder hereof, as such, any
of the rights of a stockholder of the Company, including, without limitation,
any right to vote for the election of directors or upon any matter submitted to
stockholders at any meeting thereof, or to give or withhold consent to any
corporate action, or to receive notice of meetings or other actions affecting
stockholders (except as provided in the Rights Agreement), or to receive
dividends or other distributions or subscription rights, or otherwise, until the
Right or Rights

                                      -2-

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evidenced by this Right Certificate shall have been exercised as provided in
accordance with the provisions of the Rights Agreement.

                                      -3-

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         This Right Certificate shall not be valid or obligatory for any purpose
until it shall have been countersigned by the Rights Agent.

         WITNESS the facsimile signature of the proper officers of the Company
and its corporate seal.

Dated as of:

                                                CENTRA SOFTWARE, INC.


                                                By:  ________________________
                                                Name:
                                                Title:




Countersigned:

AMERICAN STOCK TRANSFER & TRUST COMPANY
as Rights Agent

By:______________________
     Authorized Signatory

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                     [On Reverse Side of Right Certificate]

                          FORM OF ELECTION TO PURCHASE

                   (To be executed by the registered holder if
                   such holder desires to exercise the Rights
                     represented by this Right Certificate.)


         To the Rights Agent:

                  The undersigned hereby irrevocably elects to exercise
         ____________ Rights represented by this Right Certificate to purchase
         the Preferred Shares (or other shares) issuable upon the exercise of
         such Rights and requests that certificates for such shares be issued in
         the name of:

         Please insert social security
         or other identifying number



         (Please print name and address)



                  If such number of Rights shall not be all the Rights evidenced
         by this Right Certificate, a new Right Certificate for the balance
         remaining of such Rights shall be registered in the name of and
         delivered to:

         Please insert social security
         or other identifying number



         (Please print name and address)



         Dated:___________, ____

                                                     Signature

         Signature Guaranteed:

                                      -5-

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                                     NOTICE

                  The signature on the foregoing Form of Election to Purchase
         must correspond to the name as written upon the face of this Right
         Certificate in every particular, without alteration or enlargement or
         any change whatsoever.

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