To study further the role of Ca²⁺ and protein kinase C in platelet aggregation, suspensions of aspirin-treated, ³²P-prelabled, washed pig platelets containing ADP scavenger in the buffer were stimulated by Ca²⁺ ionophore A23187 and PMA,a stimulator of protein kinase C. The results indicated that: (1) 1～20 μmol/L A23187 induced platelet aggregation,as well as the phosphorylation of 40 ku and 20 ku proteins.There were dose-respone and time-respone effects of the protein phosphorylation in A23187-induced platelet activation. (2) A23187 and PMA were synergistic in platelet aggregation and protein phosphorylation. (3)Stauroporine, a protein kinase C inhibitor, in concentration of 1 μmol/L,largely suppressed platelet aggregation and completely suppressed phosphorylation of 40 ku and 20 ku proteins induced by 20 μmol/L A23187. The results imply that Ca²⁺ mobilization alone could activate protein kinase C in platelet, and Ca²⁺-induced platelet aggregation is largely dependent on activation of protein kinase C.