Exhibit 10.5      Centrex letter of intent - 12/31/02

                                      CNEX
                                LETTER OF INTENT

     Comes Now, Emergency Filtration Productions, Inc. a Nevada Corporation
("EMFP") and Centrex Inc., an Oklahoma Corporation ("CNEX") and for their letter
of intent set forth as follows:

         That EMFP is traded on the OTCBB.

         That CNEX is traded on the OTCBB.

         That EMFP currently holds military national stocking number for the
RespAide CPR isolation mask and replacement filters and for Superstat. EMFP has
received and fulfilled orders from the U.S. Navy and the Defense Supply Center.

         That CNEX is a development-stage company that owns an exclusive
worldwide license to Single Molecule Detection, a technology that was developed
at Los Alamos National Laboratory. The technology can detect bacteria or virus
by matching the DNA of the organism. The Company entered into a research
agreement with the University of California (the "University") in early 1999 to
develop a prototype system to detect the deadly form of E.coli bacteria. The
University's research and development activities are conducted at Los Alamos
National Laboratory ("Los Alamos".)

         That CNEX and EMFP marketing efforts will overlap and the parties
believe a relationship between the parties will be mutually beneficial.

         That the parties wish to explore the possibility of joint marketing, a
joint venture, a merger and/or acquisition.

         That in order to facilitate any of the transactions listed above EMFP
and CNEX will exchange shares of their Common Stock as set forth below:

         EMFP will transfer four hundred fifty thousand shares of its Common
Stock to CNEX. These shares will have piggy-back registration rights and will be
included in EMFP's next SB-2 registration statement.

         CNEX will transfer three hundred thousand shares of its Common Stock to
EMFP. These shares will have demand registration rights and be included in
CNEX's SB-2, which will be filed on December 30, 2002 under Rule 462(B) and will
be effective upon filing.

         The parties both acknowledge that they have been represented by Kaufman
& Associates, PLLC of Tulsa, OK. The parties by execution of this letter of
intent hereby acknowledge this representation and the conflict of interest. The
parties further waive any conflict of interest, whether real, perceived or
otherwise and release Kaufman & Associates, PLLC from any and all liabilities
which directly or indirectly relate to this letter of intent or any potential
joint marketing, a joint venture, a merger and/or acquisition.

         The parties further acknowledge that Kaufman & Associates has advised
both EMFP and CNEX to seek independent counsel.

                  Parties Representations and Warranties. The Parties hereby
represents and warrants to and agrees that:

     (a) Information on CNEX and EMFP. The Parties have been furnished or have
obtained from the EDGAR Website of the Securities and Exchange Commission (the
"Commission) the each other's Form 10-KSB for the year ended December 31, 2001
as filed with the Commission on April 1, 2002, together with all subsequently
filed Forms 10-Q, 8-K, and filings made with the Commission available at the
EDGAR website (hereinafter referred to collectively as the "Reports"). In
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addition, the parties have received from each other such other information
concerning their operations, financial condition and other matters as they have
requested in writing (such information in writing is collectively, the " Other
Written Information"), and considered all factors the parties deem material in
deciding on the advisability of investing in the Securities.

     (b) Information on the Parties. The Parties are an "accredited investor",
as such term is defined in Regulation D promulgated by the Commission under the
Securities Act of 1933, as amended (the "1933 Act"), are experienced in
investments and business matters, have made investments of a speculative nature
and have purchased securities of United States publicly-owned companies in
private placements in the past and, with their representatives, have such
knowledge and experience in financial, tax and other business matters as to
enable the Parties to utilize the information made available by the Company to
evaluate the merits and risks of and to make an informed investment decision
with respect to the proposed purchase, which represents a speculative
investment. The Parties have the authority and are duly and legally qualified to
purchase and own the Securities. The Parties are able to bear the risk of such
investment for an indefinite period and to afford a complete loss thereof.

         The EMFP shares shall bear the following restrictive legend:

     "THE SHARES REPRESNETED BY THIS CERTIFICATE HAVE NOT BEEN REGISTERED UNDER
THE SECURITIES ACT OF 1933, AS AMENDED. THESE SHARES MAY NOT BE SOLD, OFFERED
FOR SALE, PLEDGED OR HYPOTHECATED IN THE ABSENCE OF AN EFFECTIVE REGISTRATION
STATEMENT UNDER SUCH SECURITIES ACT OR ANY APPLICABLE STATE SECURITIES LAW OR AN
OPINION OF COUNSEL REASONABLYSATISFACTORY TO CENTREX INC. THAT SUCH REGISTRATION
IS NOT REQUIRED.

         This letter of intent is only to explore the possibility of any joint
marketing, joint venture, merger and/or acquisition and does not bind either
party to any further action whatsoever.

         In witness hereof this 30th day of December 2002.

\S\ Douglas Beplate                                  \S\ Thomas Coughlin
     Douglas Beplate                                      Dr. Thomas Coughlin
     President EMFP                                       CEO CNEX