To evaluate the kinetics of acrosome reaction, sperm samples from four fertile donors were prepared by swim-up and incubated with solutions of human zonae containing 0.1, 0.15, 0.3, 0.5 and 1.0 zonae microliter-1. After 20, 40 and 60 min of incubation at 37 degrees C, aliquots were taken for evaluation of the acrosomal status. The results showed a distinct time- and dose dependence of the acrosome reaction induced by solubilized zona proteins. After 60 min of incubation in 1.0 zonae microliter-1, about 80% of the spermatozoa showed signs of acrosomal loss; about 40% were completely acrosome-reacted. In addition, zona-bound sperm showed the same ratios of acrosome-reacted spermatozoa in control experiments. The velocity of acrosome reaction was calculated by means of a double-reciprocal plot being 2.0-2.5% min-1 for completely reacted spermatozoa and those showing signs of acrosome reaction. However, both subgroups differed considerably in their constants of equilibrium (K = 2.0 ZP microliter-1 and K = 0.2 ZP microliter-1, respectively). In nonreacted and partly reacted spermatozoa results might indicate a disturbed course of acrosome reaction or possibly the existence of different subpopulations in respect of sperm competition.