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                                                                    EXHIBIT 10.4


                            PROMISSORY NOTE

$50,000.00                                                   Seattle, Washington
                                                                  April 27, 2000

The undersigned, Peter Keane ("MAKER"), promises to pay to N2H2, Inc. or order
(collectively, "HOLDER"), the principal sum of fifty thousand Dollars
($50,000.00), or so much thereof as may be advanced from time to time by the
Holder to Maker in lawful money of the United States of America, together with
interest thereon, all as hereinafter provided and upon the following agreements,
terms and conditions.

        Interest Rate. The entire principal balance of this Note shall bear
simple interest from the date of this Note until this Note is fully paid at the
rate of eight percent (8%) per annum.

        Payments. The entire principal balance of this Note and all accrued
interest shall be due and payable within eighteen (18) months. The entire
principal balance of this Note may be prepaid at any time without penalty.

        Place of Payment. All payments pursuant to this Note shall be made to
Holder at 900 Fourth Avenue, Suite 3600, Seattle, Washington 98164, or such
other address as Holder may hereafter designate from time to time.

        Default; Default Interest. If Maker fails to pay this Note in strict
accordance with its terms, or if Maker otherwise defaults under this Note, this
Note shall bear interest from the date of default by Maker until the default is
fully cured at a default interest rate ("DEFAULT RATE") equal to sixteen percent
(16%) per annum. Notwithstanding the foregoing, if Maker defaults as described
in this paragraph, at the option of Holder, this entire Note shall become
immediately due and payable. After acceleration, this Note shall bear interest
at the Default Rate.

        Applicable Law. This Note shall be construed in accordance with and
governed by the laws of the State of Washington.

        Successors and Assigns. This Note shall inure to the benefit of Holder
and any successor or assignee of Holder. This Note shall be binding upon Maker
and Maker's successors, assignees, personal representatives and heirs.

        Costs and Expenses. Maker shall pay all costs and expenses which Holder
may incur in connection with this Note because of Maker's default, including,
without limitation, the following costs: (a) reasonable attorneys' fees for
legal services incurred by Holder in connection with any default by Maker
relating to any determination of any rights or remedies of


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Holder; and (b) reasonable attorneys' fees for any actions, proceedings, reviews
or appeals therefrom which Holder may institute or in which Holder may appear or
participate.

        Liability. Each person signing this Note shall be jointly and severally
liable with respect to this Note and hereby waives demand, presentment for
payment, notice of intention to accelerate the maturity, protest and notice of
protest and nonpayment. Each such party hereby further agrees that none of the
following events (with or without notice) shall diminish, impair or otherwise
affect the liability of any such party with respect to this Note: (a) any
modification or extension of the terms of this Note; or (b) any release of any
party liable pursuant to this Note.

ORAL AGREEMENTS OR ORAL COMMITMENTS TO LOAN MONEY, EXTEND CREDIT, OR TO FORBEAR
FROM ENFORCING REPAYMENT OF A DEBT ARE NOT ENFORCEABLE UNDER WASHINGTON LAW.

                                            /s/ Peter Keane
                                            ------------------------------------
                                                Peter Keane, Maker


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