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                                                                   Exhibit 10.15

                       FOURTH LOAN MODIFICATION AGREEMENT

         This Fourth Loan Modification Agreement (this "Loan Modification
Agreement") is entered into as of May 4, 2001, by and between CENTRA SOFTWARE,
INC., a Delaware corporation with its principal place of business at 430 Bedford
Street, Lexington, Massachusetts 02173 ("Borrower") and SILICON VALLEY BANK, a
California-chartered bank, with its principal place of business at 3003 Tasman
Drive, Santa Clara, California 95054 and with a loan production office located
at One Newton Executive Park, Suite 200, 2221 Washington Street, Newton,
Massachusetts 02462, doing business under the name "Silicon Valley East"
("Bank").

1. DESCRIPTION OF EXISTING INDEBTEDNESS. Among other indebtedness which may be
owing by Borrower to Bank, Borrower is indebted to Bank pursuant to a loan
arrangement dated as of November 5, 1997, evidenced by, among other documents, a
certain Loan and Security Agreement dated as of November 5, 1997, as affected
and amended by (i) a First Loan Modification Agreement dated December 30, 1998,
(ii) a Second Loan Modification Agreement dated April 12, 1999, and (iii) a
Third Loan Modification Agreement dated December 22, 2000 (the "Loan
Agreement"). Capitalized terms used but not otherwise defined herein shall have
the same meaning as in the Loan Agreement.

Hereinafter, all indebtedness and obligations owing by Borrower to Bank shall be
referred to as the "Obligations".

2. DESCRIPTION OF COLLATERAL. Repayment of the Obligations is secured by the
Collateral as described in the Loan Agreement (together with any other
collateral security granted to Bank, the "Security Documents").

Hereinafter, the Security Documents, together with all other documents
evidencing or securing the Obligations shall be referred to as the "Existing
Loan Documents".

3. DESCRIPTION OF CHANGE IN TERMS.

  A.  Modification(s) to Loan Agreement.

     1.   The Loan Agreement shall be amended by deleting the following
          definition appearing in Section 1.1 thereof:

                    ""2000 Committed Equipment Line" means a Credit Extension of
                    up to Two Million Dollars ($2,000,000.00)."

           and inserting in lieu thereof the following:

                    ""2000 Committed Equipment Line" means a Credit Extension of
                    up to Four Million Five Hundred Thousand Dollars
                    ($4,500,000.00)."

     2.   The Loan Agreement shall be amended by deleting Section 2.1.5(d)
          thereof and inserting in lieu thereof the following:

                    "(d) In the event the Borrower prepays in whole Equipment
                    Advances made under the 2000 Committed Equipment Line, the
                    Borrower shall be additionally obligated to the Bank for a
                    prepayment fee equal to (i) Forty-Five Thousand Dollars
                    ($45,000.00) if the 2000 Committed Equipment Line is prepaid
                    on or before December 22, 2001, and (ii) Twenty-Two Thousand
                    Five Hundred

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                    Dollars ($22,500.00) if the 2000 Committed Equipment Line is
                    prepaid after December 22, 2001 but on or before December
                    22, 2003."

     3.   Notwithstanding the terms of the Loan Agreement, Bank hereby agrees
          that in connection with the initial Equipment Advance made by Bank (in
          an amount determined by Bank) on or after the date of this Loan
          Modification Agreement, invoices submitted to Bank for financing may
          have invoice dates from and after January 1, 2001.

4. FEE. Borrower shall pay to Bank on demand all fees (including all attorneys'
fees), costs, and expenses incurred by Bank in connection with the execution and
delivery of this Loan Modification Agreement.

5. RATIFICATION OF NEGATIVE PLEDGE Borrower hereby ratifies, confirms and
reaffirms, all and singular, the terms and conditions of a certain Negative
Pledge Agreement dated as of November 5, 1997 between Borrower and Bank, and
acknowledges, confirms and agrees that said Negative Pledge Agreement shall
remain in full force and effect.

6. ADDITIONAL COVENANTS: RATIFICATION OF PERFECTION CERTIFICATE. Borrower shall
not, without providing the Bank with prior written notice: (i) except as
disclosed in the Perfection Certificate (defined below), relocate its principal
executive office or add any new offices or business locations or keep any
Collateral in any additional locations, or (ii) change its state of formation,
or (iii) change its organizational structure, (iv) change its legal name, or (v)
change any organizational number (if any) assigned by its state of formation. In
addition, the Borrower hereby certifies that no Collateral is in the possession
of any third party bailee (such as at a warehouse). In the event that Borrower,
after the date hereof, intends to store or otherwise deliver the Collateral to
such a bailee, then Borrower shall receive the prior written consent of Bank and
such bailee must acknowledge in writing that the bailee is holding such
Collateral for the benefit of Bank. Borrower hereby ratifies, confirms and
reaffirms, all and singular, the terms and disclosures contained in a certain
Perfection Certificate dated as of December 22, 2000 between Borrower and Bank
(the "Perfection Certificate"), and acknowledges, confirms and agrees the
disclosures and information above Borrower provided to Bank in the Perfection
Certificate have not changed, as of the date hereof.

7. AUTHORIZATION TO FILE. Borrower hereby authorizes Bank to file financing
statements without notice to Borrower, with all appropriate jurisdictions, as
Bank deems appropriate, in order to further perfect or protect Bank's interest
in the Collateral.

8. CONCERNING REVISED ARTICLE 9 OF THE UNIFORM COMMERCIAL CODE. The Borrower
affirms and reaffirms that notwithstanding the terms of the Security Documents
to the contrary, that the definition of "Code", "UCC" or "Uniform Commercial
Code" as set forth in the Security Documents shall be deemed to mean and refer
to "the Uniform Commercial Code as adopted by The Commonwealth of Massachusetts
(presently, Mass. Gen. Laws. Ch. 106), may be amended and in effect from time to
time. In connection therewith, the Collateral shall include, without limitation,
the following categories of assets as defined in the Code: goods (including
inventory, equipment and any accessions thereto), instruments (including
promissory notes), documents, accounts (including health-care-insurance
receivables, and license fees), chattel paper (whether tangible or electronic),
deposit accounts, letter-of-credit rights (whether or not the letter of credit
is evidenced by a writing), commercial tort claims, securities and all other
investment property, general intangibles (including payment intangibles and
software) but excluding intellectual property, as set forth in the Loan
Agreement, supporting obligations and any and all proceeds of any thereof,
wherever located, whether now owned or hereafter acquired.

9. CONSISTENT CHANGES. The Existing Loan Documents are hereby amended wherever
necessary to reflect the changes described above.

10. RATIFICATION OF LOAN DOCUMENTS. Borrower hereby ratifies, confirms, and
reaffirms all terms and conditions of all security or other collateral granted
to the Bank, and confirms that the indebtedness secured thereby includes,
without limitation, the Obligations.

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11. NO DEFENSES OF BORROWER. Borrower agrees that, as of this date, it has no
defenses against the obligations to pay any amounts under the Obligations.

12. CONTINUING VALIDITY. Borrower understands and agrees that in modifying the
existing Obligations, Bank is relying upon Borrower's representations,
warranties, and agreements, as set forth in the Existing Loan Documents. Except
as expressly modified pursuant to this Loan Modification Agreement, the terms of
the Existing Loan Documents remain unchanged and in full force and effect.
Bank's agreement to modifications to the existing Obligations pursuant to this
Loan Modification Agreement in no way shall obligate Bank to make any future
modifications to the Obligations. Nothing in this Loan Modification Agreement
shall constitute a satisfaction of the Obligations. It is the intention of Bank
and Borrower to retain as liable parties all makers of Existing Loan Documents,
unless the party is expressly released by Bank in writing. No maker will be
released by virtue of this Loan Modification Agreement.

13. RIGHT OF SET-OFF. In consideration of Bank's agreement to enter into this
Loan Modification Agreement, Borrower and any guarantor hereby grant to Bank, a
lien, security interest and right of setoff as security for all Obligations to
Bank, whether now existing or hereafter arising upon and against all deposits,
credits, collateral and property, now or hereafter in the possession, custody,
safekeeping or control of Bank or any entity under the control of Silicon Valley
Bank or in transit to any of them. At any time after the occurrence and during
the continuance of an Event of Default, without demand or notice, Bank may set
off the same or any part thereof and apply the same to any liability or
obligation of Borrower and any guarantor even though unmatured and regardless of
the adequacy of any other collateral securing the loan. ANY AND ALL RIGHTS TO
REQUIRE BANK TO EXERCISE ITS RIGHTS OR REMEDIES WITH RESPECT TO ANY OTHER
COLLATERAL WHICH SECURES THE OBLIGATIONS, PRIOR TO EXERCISING ITS RIGHT OF
SETOFF WITH RESPECT TO SUCH DEPOSITS, CREDITS OR OTHER PROPERTY OF THE BORROWER
OR ANY GUARANTOR, ARE HEREBY KNOWINGLY, VOLUNTARILY AND IRREVOCABLY WAIVED.

14. JURISDICTION/VENUE. Borrower accepts for itself and in connection with its
properties, unconditionally, the non-exclusive jurisdiction of any state or
federal court of competent jurisdiction in the Commonwealth of Massachusetts in
any action, suit, or proceeding of any kind against it which arises out of or by
reason of this Loan Modification Agreement; provided, however, that if for any
reason Bank cannot avail itself of the courts of the Commonwealth of
Massachusetts, then venue shall lie in Santa Clara County, California.

15. COUNTERSIGNATURE. This Loan Modification Agreement shall become effective
only when it shall have been executed by Borrower and Bank (provided, however,
in no event shall this Loan Modification Agreement become effective until signed
by an officer of Bank in California).

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         This Loan Modification Agreement is executed as a sealed instrument
under the laws of the Commonwealth of Massachusetts as of the date first written
above.

BORROWER:                             BANK:

CENTRA SOFTWARE, INC.                 SILICON VALLEY BANK, doing business as
                                      SILICON VALLEY EAST


By:_____________________________      By:___________________________________

Name:___________________________      Name:_________________________________

Title:__________________________      Title:________________________________


                                      SILICON VALLEY BANK

                                      By:___________________________________

                                      Name:_________________________________

                                      Title:________________________________

                                      (signed in Santa Clara County, California)



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