Methods are described for the assay of chloroform-activated and fibrinolysin-activated plasma proteolytic enzyme, and for the determination of the inhibitory activity of plasma or other substances against chloroform-activated enzyme. The inhibitory activities of plasma, serum, and crude plasma albumin against chloroform-activated plasma proteolytic enzyme were proportional to the logarithm of the concentration of the inhibitory substance tested. This suggests that enzyme and inhibitor react in accordance with the law of mass action. The rate of fibrinolysis of recalcified plasma clots could not be related to the total proteolytic activity available in the plasma, nor to the inhibitory activity of fresh plasma or serum against plasma proteolytic enzyme. During the incubation of a recalcified plasma clot at 37 degrees C., the inhibitory activity of its serum against plasma proteolytic enzyme decreased until a minimal stationary level was reached. The clot lysis time could be correlated directly with the time which elapsed until this minimal level was reached.