Sperm samples from 29 men randomly selected from the andrology laboratory, were used to evaluate acrosome reaction response to solubilized human zona pellucida. Capacitated sperm samples were exposed to a solution containing 2 zona pellucidae (ZP) per microl for 60 min, after which acrosomal status were recorded using a PSA-FITC technique. Controls included samples supplied by fertile sperm donors. After completion of acrosome reaction studies, patient samples were divided according to the percentage of morphologically normal spermatozoa. Three basic groups were identified, namely, fertile donors, teratozoospermic (normal sperm morphology 5-14%; n = 25) and severely teratozoospermic (normal sperm morphology < 4%; n = 4) groups. The mean percent normal sperm were 15.8 +/- 0.9, 10.4 +/- 0.7 and 2.7 +/- 0.7, respectively, for normozoospermic donors, teratozoospermic and severely teratozoospermic men. The mean percentage (+/-SE) ZP mediated acrosome reacted sperm among teratozoospermic and severely teratozoospermic cases was 25.8% +/- 0.9 and 19.0% +/- 0.9 (P = 0.001), compared to 36.8% +/- 0.9 for the donor controls. Results were analysed and expressed as correlations between sperm morphology and acrosomal response to human solubilized zona pellucida, spontaneous and calcium ionophore induced acrosome reaction. Predictive values for acrosome responsiveness were depicted with ROC curve analyses. Sperm morphology evaluated by strict criteria correlated positively and highly significantly with the responsiveness of the acrosome reaction (r = 0.91, P = 0.0001). At a morphology cut-off value of 4%, the ROC curve analysis showed sperm morphology to be highly predictive of zona pellucida induced acrosome responsiveness with a sensitivity of 100% and negative predictive value of 100%. Spontaneous and calcium ionophore induced acrosome reactions revealed no correlation with sperm morphology. It was concluded that (i) morphological features of human spermatozoa are indicative of specific functional characteristics; (ii) zona pellucida induction of the acrosome reaction is superior, as a predictor of sperm morphology, compared to calcium ionophore induced and spontaneous acrosome reactions.