The kinetics of the reaction between human plasma kallikrein and C1-esterase inhibitor was studied in a purified system. By monitoring the inhibition reaction for extended periods of time, it was found to proceed in two consecutive steps, a fast reversible second-order binding step followed by a slower, irreversible first-order transition. The rate constants in this reaction model were determined, as well as the dissociation constant of the initial, reversible enzyme-inhibitor complex. Thus, at 37 degrees C the second-order rate constant k1 was found to be 5 X 10(4) M-1 . s-1, the first-order rate constant k2 was 5 X 10(-4) s-1, and the dissociation constant K was 1.5 X 10(-8) M. Heparin (28 U/ml) and 6-aminohexanoic acid (10 mM) had no effect on the k1 of the reaction.