Calcium and phospholipid-dependent protein kinase (PKC) activity was detected mainly in the cytosol of the mouse sperm. The PKC in the cytosol fraction was partially purified by ion-exchange chromatography. Using the partially purified PKC, the phosphorylation of PKC substrates was examined in vitro. The phosphorylation of the 80 kDa protein was enhanced by phorbol ester treatment in vitro as well as in vivo. The partial amino acid sequence of this protein was homologous with that of guanosine 5'-cyclic monophosphate (cGMP)-dependent protein kinase and myosin light chain kinase, both of which are related to ligand-receptor-transduction. The present data suggest that the activation of PKC and subsequent specific protein phosphorylatioin might be involved in the regulation of the zona pellucida-induced acrosome reaction.