By means of a triple-stain technique, the acrosome reaction in infertile and fertile males was investigated. The highest correlations with the percentage of acrosome-reacted sperm were found for motile sperm concentration (r = .60, p < .01) and percent motility (r = .58, p < .05). Significant correlations were also demonstrated between acrosome reactivity and both sperm concentration (r = .50, p < .05) and velocity (r = .40, p < .05). There was only a weak correlation between the percentage of acrosome-reacted sperm and that of swollen sperm. The ratio of acrosome-reacted sperm demonstrated in oligoasthenozoospermic patients was significantly lower than that observed in normozoospermic or fertile males (both, p < .05). There was no significant difference in the ratio of acrosome-reacted sperm between normozoospermic and fertile males. However, all the cases in the latter group had 15% or more acrosome-reacted sperm, whereas the parameter was widely distributed in the former group and some in this group had a decreased ratio. It would appear that a disturbance of the acrosome reaction is one of the causes of reduced fertility potential in normozoospermic males.