We used flow cytometry and two anti-CD9 murine monoclonal antibodies (NNKY1-19, MALL13) to investigate the glycoprotein composition and the potential functions of microparticles (MP) released by platelets exposed to these antibodies in vitro. NNKY1-19 produced aggregation with characteristics similar to those noted in previous reports. The action of MALL13 on platelets in platelet-rich plasma (PRP), however, differs from that of other anti-CD9 antibodies. The normal fluctuation in the MALL13-induced change in optical density disappeared when complement was present. MALL13-induced effect for platelet in PRP was not inhibited by preincubation with monoclonal anti-GPIIb/IIIa antibody, but was inhibited in washed platelets (WP). Furthermore, following MALL13 stimulation in PRP platelets, the amount of buffer LDH markedly increased and electron microscopy findings showed vacuoles appearing inside the platelets. These results suggest that MALL13 has at least two effects on platelets that differ for PRP platelets and WP. The number of MP released was increased by the addition of anti-CD9 antibodies. MP surfaces were found to be rich in CD9 protein. MALL13 stimulation lead to a significant increase in the binding of C1q and C3 to platelets and caused the production of MP to occur more rapidly than it did the exposure of fibrinogen binding sites in the presence of complement. The analysis of the relationship of MP to anti-CD9 monoclonal antibody may be useful in the investigation of the relationship between platelet function and coagulation regulation.