Eight monoclonal antibodies were obtained which recognized platelet surface antigens of these, 5 (NNKYl-32, NNKY2-5, NNKY2-6, NNKY2-11, NNKY2-18 ) recognized GP IIb-IIIa complex, 2 (NNKY5-4, NNKY5-5 ) recognized GP lb and 1 (NNKYl-19) recognized CD 9 antigen. They were used to research the platelet membrane antigens.Monoclonal antibodies that recognize CD 9 antigen, which exists on the surface of platelets, acute lymphoblastic leukemia cells, eosinophils and other tissue, are known to act as an aggregating agent to platelets and NNKYl-19 was fond to induce platelet aggregation accompanied by ATP release. NNKY5-4 had no effect on platelet functions. NNKY5-5 inhibited aggregation induced by ristocetin but had no effect on aggregation induced by ADP, collagen, thrombin, and NNKYl-19. NNKYl-32, 2-5, 2-6, 2-11, and 2-18 inhibited aggregation induced by ADP, collagen, thrombin, and NNKYl-19, although slight release of ATP was recognized when NNKYl-19-induced aggregation was completely inhibited by NNKYl-32. Mutual inhibition of binding to platelet membranes between the 3 groups of monoclonal antibodies was not recognizedNNKYl-19-induced aggregation was associated with a lag time that was plo-longed in inverse proportion to antibody concentration. Aspirin had almost no effect on NNKYl-19-induced aggregation. A TXA2 receptor antagonist, a calci-um-channel blocking drug and EDTA inhibited NNKYl-19-induced aggregation. These results indicate that GP I b, GP IIb-IIIa complex and the cyclooxygenase pathway are not involved in NNKYl-19-induced platelet activation, that the target of NNKYl-19 on the platelet membrane is same as that of TXA2, and that the mechanism of activation by NNKYl-19 is related to calcium flux.