The acrosome reaction was determined in aliquots from ejaculates of 74 patients undergoing in-vitro fertilization at the University of Giessen, Germany, by means of the triple-stain technique. The percentage of acrosome-reacted spermatozoa after low-temperature induction of the acrosome reaction was not significantly related with the fertilization rate (H test, P = 0.693, SJ test, P = 0.366). However, all patients showing < 13.0% acrosome-reacted spermatozoa had poor fertilization rates. Highly significant differences between patients could be detected by correlating the inducibility of the acrosome reaction with the fertilization rate (H test, P = 0.018; SJ test, P = 0.004); patients with high fertilization rates showed a corresponding high inducibility of acrosome reactions. From our results, it is evident that percentages of acrosome-reacted spermatozoa < 13.0% or an inducibility of the acrosome reaction of < 7.5% are indicative of subfertility.