<PAGE>
 
                                                                  EXHIBIT 10.6

                                PROMISSORY NOTE
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<CAPTION>

<S>                <C>          <C>           <C>        <C>     <C>          <C>       <C>        <C>
 Principal          Loan Date     Maturity    Loan No    Call    Collateral   Account   Officer    Initials
$2,500,000.00      09-01-1998   09-01-1999     9003       4AO      9093       9240904    60683
References in the shaded area are for Lender's use only and do not limit the applicability of this document to any particular loan 
or item.
</TABLE>


<TABLE>
<CAPTION>

<S>                                                        <C>
Borrower: Wave Technologies International, Inc.            Lender: COMMERCE BANK, N.A.
          (TIN: 43-1481443)                                        8000 Forsyth
          10845 Olive Blvd                                         PO Box 11573
                         St. Louis, MO 63141                                      St. Louis, MO 63105-0373

===================================================================================================================================
</TABLE>

<TABLE>
<CAPTION>

<S>                                        <C>                           <C>   
Principal Amount:  $2,500,000.00           Initial Rate:  8.500%         Date of Note: September 1, 1998
</TABLE>


PROMISE TO PAY. Wave Technologies International, Inc. ("Borrower") promises to 
pay to COMMERCE BANK, N.A. ("LENDER"), or order, in lawful money of the United 
States of America, the principal amount of Two Million Five Hundred Thousand & 
00/100 Dollars ($2,500,000.00) or so much as may be outstanding, together with 
interest on the unpaid outstanding principal balance of each advance. Interest 
shall be calculated from the date of each advance until repayment of each 
advance.

PAYMENT. Borrower will pay this loan in one payment of all outstanding principal
plus all accrued unpaid interest on September 1, 1999. In addition, Borrower
will pay regular monthly payments of accrued unpaid interest beginning October
1, 1998, and all subsequent interest payments are due on the same day of each
month after that. The annual interest rate for this Note is computed on a
365/360 basis; that is, by applying the ratio of the annual interest rate over a
year of 360 days, multiplied by the outstanding principal balance, multiplied by
the actual number of days the principal balance is outstanding. Borrower will
pay Lender at Lender's address shown above or at such other place as Lender may
designate in writing. Unless otherwise agreed or required by applicable law,
payments will be applied first to accrued unpaid interest, then to principal,
and any remaining amount to any unpaid collection costs and late charges.

VARIABLE INTEREST RATE. The interest rate on this Note is subject to change from
time to time based on changes in an index which is the per annum rate from time 
to time announced by Lender at its main office as the prime rate, or as the case
may be, the base, reference or other rate then in use for commercial loan 
reference purposes, not necessarily the lowest or even favored rate, which 
serves as the basis upon which effective rates of interest are calculated for 
those loans making reference thereto (the "Index"). The initial rate and current
index described above are based on information available as of the date of 
preparation of this note and is subject to change if there is any change in the 
Index between the note preparation date and the Loan Date and Date of Note 
recited above. Lender will tell Borrower the current index rate upon Borrower's 
request. Borrower understands that Lender may make loans based on other rates as
well. The interest rate change will not occur more often than each Day. Rates of
interest tied to the Index shall change with and be effective on the date of 
each change in the Index. The Index currently is 8.500% per annum. The Interest 
rate to be applied to the unpaid principal balance of this Note will be at a 
rate equal to the Index, resulting in an initial rate of 8.500% per annum. 
NOTICE: Under no circumstances will the Interest rate on this Note be more than 
the maximum rate allowed by applicable law.

PREPAYMENT. Borrower agrees that all loan fees and other prepaid finance charges
are earned fully as of the date of the loan and will not be subject to refund
upon early payment (whether voluntary or as a result of default), except as
otherwise required by law. Except for the foregoing, Borrower may pay without
penalty all or a portion of the amount owed earlier than it is due. Early
payments will not, unless agreed to by Lender in writing, relieve Borrower of
Borrower's obligation to continue to make payments of accrued unpaid interest.
Rather, they will reduce the principal balance due.

DEFAULT. Borrower will be in default if any of the following happens: (a)
Borrower fails to make any payment when due. (b) Borrower breaks any promise
Borrower has made to Lender, or Borrower fails to comply with or to perform when
due any other term, obligation, covenant, or condition contained in this Note or
any agreement related to this Note, or in any other agreement or loan Borrower
has with Lender. (c) Any representation or statement made or furnished to Lender
by Borrower or on Borrower's behalf is false or misleading in any material
respect either now or at the time made or furnished. (d) Borrower becomes
insolvent, a receive is appointed for any part of Borrower's property, Borrower
makes an assignment for the benefit of creditors, or any proceeding is commenced
either by Borrower or against Borrower under any bankruptcy or insolvency laws.
(e) Any creditor tries to take any of Borrower's property on or in which Lender
has a lien or security interest. This includes a garnishment of any of
Borrower's accounts with Lender. (f) Any guarantor dies or any of the other
events described in this default section occurs with respect to any guarantor of
this Note or any guarantor seeks, claims or otherwise attempts to limit, modify
or revoke such guarantor's guarantee of this Note. (g) A material adverse change
occurs in Borrower's financial condition, or Lender believes the prospect of
payment or performance of the Indebtedness is impaired. (h) Lender in good faith
deems itself insecure.

LENDER'S RIGHTS. Upon default, Lender may declare the entire unpaid principal 
balance on this Note and all accrued unpaid interest immediately due, without 
notice, and then Borrower will pay that amount. Upon default, including failure 
to pay upon final maturity, Lender, at its option, may also, if permitted under 
applicable law, do one or both of the following: (a) increase the variable 
interest rate on this Note to 3.000 percentage points over the index, and (b) 
add any unpaid accrued interest to principal and such sum will bear interest 
therefrom until paid at the rate provided in this Note (including any increased 
rate). The interest rate will not exceed the maximum rate permitted by 
applicable law. Lender may hire or pay someone else to help collect this Note if
Borrower does not pay. Borrower also will pay Lender that amount. This includes,
subject to any limits under applicable law, Lender's attorneys' fees and 
Lender's legal expenses whether or not there is a lawsuit, including attorneys' 
fees and legal expenses for bankruptcy proceedings (including efforts to modify 
or vacate any automatic stay or injunction), appeals, and any anticipated 
post-judgment collection services. If not prohibited by applicable law, Borrower
also will pay any court costs, in addition to all other sums provided by law. 
This Note has been delivered to Lender and accepted by Lender in the State of 
Missouri. If there is a lawsuit, Borrower agrees upon Lender's request to submit
to the jurisdiction of the courts of St. Louis County, the State of Missouri. 
This Note shall be governed by and construed in accordance with the laws of the 
State of Missouri.

RIGHTS OF SETOFF. Borrower grants to Lender a contractual security interest in,
and hereby assigns, conveys, delivers, pledges, and transfers to Lender all
Borrower's right, title and interest in and to, Borrower's accounts with Lender
(whether checking, savings, or some other account), including without limitation
all accounts held jointly with someone else and all accounts Borrower may
open in the future, excluding however all IRA and Keogh accounts, and all trust
accounts for which the grant of a security interest would be prohibited by law.
Borrower authorizes Lender, to the extent permitted by applicable law, to charge
or setoff all sums owing on this Note against any and all such accounts.

LINE OF CREDIT. This Note evidences a revolving line of credit. Advances under
this Note may be requested orally by Borrower or by an authorized person. Lender
may, but need not, require that all oral requests be confirmed in writing. All
communications, instructions, or directions by telephone or otherwise to Lender
are to be directed to Lender's office shown above. The following party or
parties are authorized to request advances under the line of credit until Lender
receives from Borrower at Lender's address shown above written notice of
revocation of their authority; Kenneth W. Kousky, President and J. M. Bowles,
CFO. Borrower agrees to be liable for all sums either: (a) advanced in
accordance with the instructions of an authorized person or (b) credited to any
of Borrower's accounts with Lender. The unpaid principal balance owing on this
Note at any time may be evidenced by endorsements on this Note or by Lender's
internal records, including daily computer print-outs. Lender will have no
obligation to advance funds under this Note if: (a) Borrower or any guarantor is
in default under the terms of this Note or any agreement that Borrower or any
guarantor has with Lender, including any agreement made in connection with the
signing of this Note; (b) Borrower or any guarantor ceases doing business or is
insolvent; (c) any guarantor seeks, claims or otherwise attempts to limit,
modify or revoke such guarantor's guarantee of this Note or any other loan with
Lender; (d) Borrower has applied funds provided pursuant to this Note for
purposes other than those authorized by Lender; or (e) Lender in good faith
deems itself insecure under this Note or any other agreement between Lender and
Borrower.

PRIOR NOTE: "This is a renewal of Borrower's note dated September 1, 1997." 
ALSO: This note is subject to the terms and conditions of the General Loan and 
Security Agreement dated August 31, 1995, and amended January 5, 1996 and 
September 1, 1996, and amended August 9, 1998.

GENERAL PROVISIONS: Lender may delay or forego enforcing any of its rights or
remedies under this Note without losing them. Borrower and any other person who
signs, guarantees or endorses this Note, to the extent allowed by law, waive
presentment, demand for payment, protest and notice of dishonor. Upon any change
in the terms of this Note, and unless otherwise expressly stated in writing, no
party who signs this Note, whether as maker, guarantor, accommodation maker or
endorser, shall be released from liability. All such parties agree that Lender
may renew or extend (repeatedly and for any length of time) this loan, or
release any party or guarantor or collateral; or impair, fail to realize upon
or perfect Lender's security interest in the collateral; and take any other
action deemed necessary by Lender without the consent of or notice to anyone.
all such parties also agree that Lender may modify this loan without the consent
of or notice to anyone other than the party with whom the modification is made.



<PAGE>
 
09-01-1998                      PROMISSORY NOTE                           Page 2
                                  (Continued)
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Jury Waiver. Lender and Borrower hereby waive the right to any jury trial in any
action, proceeding, or counterclaim brought by either Lender or Borrower against
the other.

PRIOR TO SIGNING THIS NOTE, BORROWER READ AND UNDERSTOOD ALL THE PROVISIONS OF 
THIS NOTE, INCLUDING THE VARIABLE INTEREST RATE PROVISIONS. BORROWER AGREES TO 
THE TERMS OF THE NOTE AND ACKNOWLEDGES RECEIPT OF A COMPLETED COPY OF THE NOTE.

ORAL AGREEMENTS OR COMMITMENTS TO LOAN MONEY, EXTEND CREDIT OR TO FOREBEAR FROM 
ENFORCING REPAYMENT OF A DEBT INCLUDING PROMISES TO EXTEND OR RENEW SUCH DEBT 
ARE NOT ENFORCEABLE. TO PROTECT YOU (BORROWER(S)) AND US (CREDITOR) FROM 
MISUNDERSTANDING OR DISAPPOINTMENT, ANY AGREEMENTS WE REACH COVERING SUCH 
MATTERS ARE CONTAINED IN THIS WRITING, WHICH IS THE COMPLETE AND EXCLUSIVE 
STATEMENT OF THE AGREEMENT BETWEEN US, EXCEPT AS WE MAY LATER AGREE IN WRITING 
TO MODIFY IT.

BORROWER:

Wave Technologies International, Inc.

   
By: J.M. Bowles
    -----------------------------
    J.M. Bowles, CFO

================================================================================
Variable Rate. Line of Credit.      LASER PRO. Reg. U.S. Pat. & T.M. Off., Ver.
                                    S.26(c) 1998 CFI ProServices, Inc. All
                                    rights reserved. (MO-D20 80001920.LN)