The acrosomal reaction of human spermatozoa was assessed, according to a triple-stain technique, at the end of a 17-hour capacitation period in the search for a predictive test of sperm efficiency in an in vitro fertilization program. A 400% increase in the acrosomal reaction rate of the living spermatozoa was observed (4.2% +/- 3.2% before incubation versus 16% +/- 11% after incubation). No correlation could be evidenced between the abilities for spermatozoa to react and to fertilize mature human oocytes in vitro, whatever the initial semen quality. The acrosome reaction rate was not enhanced by the presence of the cumulus oocyte complex, by the selection of motile spermatozoa, or by the increase in albumin concentration of the culture medium.