Mouse caput spermatozoa are considered immature and thus unable to fertilize oocytes. In this study, we determined whether washing mouse caput spermatozoa increased their ability to acrosome react in response to a physiological stimulus. The results obtained showed that mouse caput spermatozoa incubated in Earles' modified medium containing calcium chloride and supplemented with BSA and pyruvate for 1 h at 37 degrees C and then washed acrosome reacted in response to both solubilized zonae and immunoaggregation of a zona binding site. In addition, the material removed from caput spermatozoa by washing blocked induced acrosome reactions of cauda spermatozoa. The data indicate that mouse caput epididymal spermatozoa, if incubated and washed, can undergo physiological acrosome reactions.