Acrosin activity (aa) was routinely measured in 189 infertile men who attended the hospital between March 1984 and January 1985. The evaluation of the male included semen analysis, microbial screening, postcoital testing (PCT), in vitro sperm penetration test (SPMT), and screening for anti-sperm antibodies in serum. The strongest positive correlations of aa were found for sperm motility, count, morphology, vitality and volume. In presence of a poor PCT result, aa was significantly reduced, even in patients with good motility in native semen. A similar result was obtained with the SPMT: reduced sperm density and poor sperm motility in the capillary after 2 hours were significantly correlated to reduced aa. Smokers exhibited lower aa in presence of normal sperm count and motility. In the infertile group aa was significantly lower than in the group of men whose spouses conceived. On the basis of our results, the routine determination of aa is not necessary, but it provides additional prognostic information in couples with unexplained infertility.