<PAGE>

                                                                   EXHIBIT 10.23


                                                                         Emerald






                                December 1, 2000



Mr. Richard A. Parlontieri
Emissions Testing, Inc.
400 Colony Park
Building 104, Suite 600
Cumming, Georgia  30041

          Re:  Letter Agreement Regarding 310,000 Shares of the Common Stock of
               Emissions Testing, Inc., a Georgia Corporation (the "Company"),
               Owned of Record by Emerald Marine, Ltd. ("Emerald Marine")

Dear Mr. Parlontieri:

     As you know, Emerald Marine is the owner of record of 310,000 shares of the
common stock, no par value per share, of the Company.

     The Company has filed a registration statement on Form S-1 (the
"Registration Statement") with the Securities and Exchange Commission (the
"SEC") for the registration of certain shares of the Company's common stock
("Common Stock"), in a public offering (the "Offering") under the Securities Act
of 1933, as amended (the "Act"). Among the shares of Common Stock to be
registered in the Offering are 100,000 shares of Common Stock owned of record by
Emerald Marine ( the "Registered Shares"). The balance of the shares of Common
Stock owned by Emerald Marine, I.E., 210,000 shares, will not be registered with
the SEC by the Company in the Offering (the "Restricted Shares").

     Inasmuch as Emerald Marine wishes to induce the Company to continue its
efforts in connection with the Offering, Emerald Marine hereby agrees with, and
represents to, the Company that Emerald Marine will not, directly or indirectly,
sell, offer to sell, grant an option for the sale of, assign, transfer, pledge,
hypothecate or otherwise encumber or dispose of in any manner, any of the
Registered Shares (or any interest therein) or any of the Restricted Shares (or
any interest therein), INCLUDING, WITHOUT LIMITATION, PURSUANT TO RULE 144 OR
RULE 144A UNDER THE ACT, from and after the date hereof except as follows: The
Company agrees that Emerald Marine shall have the right to sell, offer to sell,
grant an option for the sale of, assign, transfer, pledge, hypothecate or
otherwise encumber and/or dispose of in any manner, all or any part of the
Registered Shares and all or any part of the Restricted Shares at any time after
the earlier of (I) 12:01 AM (Atlanta, Georgia time) on October 1, 2001, or (II)
the date and time that the

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Emissions Testing, Inc.
December 1, 2000
Page 2


Company has reduced to $250,000 or below the outstanding aggregate principal
amount of the Company's indebtedness to GCA Strategic Investment Fund Limited, a
Bermuda corporation ("GCA") (or its assignees) under the convertible debentures
issued and sold (before or after the date hereof) by the Company pursuant to the
terms of that certain Securities Purchase Agreement dated effective as of
June 1, 2000 between the Company and GCA, as the same may be amended from time
to time.

     Emerald Marine agrees and understands that:

          (a)  Any sale or other transaction involving all or any part of
               Registered Shares and all or any part of the Restricted Shares
               are, and shall hereafter remain, subject to all applicable
               federal and state securities laws and regulations.

          (b)  The Company may place a legend reflecting this letter and terms
               hereof on any certificate evidencing all or any part of the
               shares described herein, and may give written notice of this
               letter, and the terms hereof, to the Company's transfer agent;

          (c)  This letter, and the terms hereof, shall be binding upon Emerald
               Marine and the successors and assigns of Emerald Marine; and

          (d)  The Company is relying upon the agreements and representations of
               Emerald Marine set forth in this letter in filing the
               Registration Statement and amendments thereto and in consummating
               the Offering.

     The Company agrees that, upon the occurrence of the event described in
clause (II) above, the Company shall give prompt written notice thereof to
Emerald Marine at the last known address then available to the Company.

     The parties hereto agree that this Letter Agreement shall not be amended,
modified or terminated without the prior written consent of GCA, which consent
may be granted or denied in the sole discretion of GCA.



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Emissions Testing, Inc.
December 1, 2000
Page 3



         If the foregoing meets with your understanding of our agreement, please
so signify by signing and returning this Letter Agreement to us.


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<S>                                   <C>
                                      Very truly yours,

                                      EMERALD MARINE, LTD.



                                      By: /s/ G. Mundy
                                         ----------------------------------
                                        Name: G. Mundy
                                             ------------------------------
                                         Position: Director
                                                  -------------------------
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Agreed to and accepted as of the
day and year first above written.


EMISSIONS TESTING, INC.



By: /s/ Rich Parlontieri
    ------------------------------
   Name: Rich Parlontieri
         -------------------------
   Position: President/CEO
            ----------------------