Abstract: A method for assaying body fluid samples containing heparin and a diagnostic kit are described. Since the reactions go to completion, timing of the assay is not required. The sample is mixed with a heparin-dependent protease inhibitor and either a heparin-independent irreversible inhibitor or a protease substrate. The coagulation enzyme (protease) is then added in a limiting quantity and it either distributes between the heparin-dependent inhibitor and the heparin-indepedent irreversible inhibitor, or the heparin-dependent inhibitor and the protease substrate. The distribution pattern of complex formation of the protease with the two inhibitors or the level of product of the protease-catalyzed hydrolysis of the substrate are used as measures of the heparin activity. The irreversible inhibitor is a peptidyl chloromethyl ketone and the substrate is a synthetic chromogenic or fluorogenic compound that produces a readily measured signal.