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                                                                   Exhibit 10.12

                       SUPPLEMENT NO. 1 dated as of October 14, to the
                  Subsidiary Guarantee Agreement dated as of June 23, 1999,
                  among each of the subsidiaries listed on Schedule I thereto
                  (each such subsidiary individually, a "GUARANTOR" and
                  collectively, the "GUARANTORS") of ANTEON CORPORATION, a
                  Virginia corporation (the "BORROWER"), and MELLON BANK, N.A.,
                  a national banking association, as collateral agent (the
                  "COLLATERAL AGENT") for the Secured Parties (as defined in
                  the Credit Agreement referred to below).

     A. Reference is made to the Credit Agreement dated as of June 23, 1999
(as amended, supplemented or otherwise modified from time to time, the
"CREDIT AGREEMENT"), among the Borrower, the lenders from time to time party
thereto (the "LENDERS"), Credit Suisse First Boston, as administrative agent
for the Lenders (in such capacity, the "ADMINISTRATIVE AGENT"), and as
issuing bank, Mellon, as Collateral Agent, swingline lender and syndication
agent, and Deutsche Bank AG, New York Branch, as documentation agent.

     B. Capitalized terms used herein and not otherwise defined herein shall
have the meanings assigned to such terms in the Guarantee Agreement and the
Credit Agreement.

     C. The Guarantors have entered into the Guarantee Agreement in order to
induce the Lenders to make Loans and the Issuing Bank to issue Letters of
Credit. Pursuant to Section 5.12 of the Credit Agreement, each Domestic
Subsidiary of the Borrower (other than any Inactive Subsidiary) that was not
in existence or not a Domestic Subsidiary on the date of the Credit Agreement
is required to enter into the Guarantee Agreement as a Guarantor upon
becoming a Domestic Subsidiary (or ceasing to be an Inactive Subsidiary).
Section 20 of the Guarantee Agreement provides that additional Domestic
Subsidiaries of the Borrower may become Guarantors under the Guarantee
Agreement by execution and delivery of an instrument in the form of this
Supplement. The undersigned Domestic Subsidiary of the Borrower (the "NEW
GUARANTOR") is executing this Supplement in accordance with the requirements
of the Credit Agreement to become a Guarantor under the Guarantee Agreement
in order to induce the Lenders to make additional Loans and the Issuing Bank
to issue additional Letters of Credit and as consideration for Loans
previously made and Letters of Credit previously issued.

     Accordingly, the Collateral Agent and the New Guarantor agree as follows:


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     SECTION 1. In accordance with Section 20 of the Guarantee Agreement, the
New Guarantor by its signature below becomes a Guarantor under the Guarantee
Agreement with the same force and effect as if originally named therein as a
Guarantor and the New Guarantor hereby (a) agrees to all the terms and
provisions of the Guarantee Agreement applicable to it as a Guarantor
thereunder and (b) represents and warrants that the representations and
warranties made by it as a Guarantor thereunder are true and correct on and
as of the date hereof. Each reference to a "Guarantor" in the Guarantee
Agreement shall be deemed to include the New Guarantor. The Guarantee
Agreement is hereby incorporated herein by reference.

     SECTION 2. The New Guarantor represents and warrants to the Collateral
Agent and the other Secured Parties that this Supplement has been duly
authorized, executed and delivered by it and constitutes its legal, valid and
binding obligation, enforceable against it in accordance with its terms.

     SECTION 3. This Supplement may be executed in counterparts, each of
which shall constitute an original, but all of which when taken together
shall constitute a single contract. This Supplement shall become effective
when the Collateral Agent shall have received counterparts of this Supplement
that, when taken together, bear the signatures of the New Guarantor and the
Collateral Agent. Delivery of an executed signature page to this Supplement
by fax transmission shall be as effective as delivery of a manually executed
counterpart of this Supplement.

     SECTION 4. Except as expressly supplemented hereby, the Guarantee
Agreement shall remain in full force and effect.

     SECTION 5. THIS SUPPLEMENT SHALL BE GOVERNED BY, AND CONSTRUED IN
ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK.

     SECTION 6. In case any one or more of the provisions contained in this
Supplement should be held invalid, illegal or unenforceable in any respect,
the validity, legality and enforceability of the remaining provisions
contained herein and in the Guarantee Agreement shall not in any way be
affected or impaired thereby (it being understood that the invalidity of a
particular provision hereof in a particular jurisdiction shall not in and of
itself affect the validity of such provision in any other jurisdiction). The
parties hereto shall endeavor in good-faith negotiations to replace the
invalid, illegal or unenforceable provisions with valid provisions the
economic effect of which comes as close as possible to that of the invalid,
illegal or unenforceable provisions.

     SECTION 7. All communications and notices hereunder shall be in writing
and given as provided in Section 14 of the Guarantee Agreement. All


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communications and notices hereunder to the New Guarantor shall be given to
it at the address set forth under its signature below, with a copy to the
Borrower.

     SECTION 8. The New Guarantor agrees to reimburse the Collateral Agent
for its out-of-pocket expenses in connection with this Supplement, including
the fees, disbursements and other charges of counsel for the Collateral Agent.

     IN WITNESS WHEREOF, the New Guarantor and the Collateral Agent have duly
executed this Supplement to the Guarantee Agreement as of the day and year
first above written.

                                       ANTEON-CITI LLC,

                                       by: /s/ Carlton B. Crenshaw
                                          -----------------------------------
                                          Name: Carlton B. Crenshaw
                                          Title: Vice President
                                          Address: 3211 Jermantown Rd.
                                                   Fairfax, VA 22030

                                       MELLON BANK, N.A., as Collateral Agent,

                                       by:  /s/ Leslie A. Grizzard
                                          -----------------------------------
                                          Name: Leslie A. Grizzard
                                          Title: Vice President