Abstract Platelet activation induced by anti-CD9 mAb, which depends upon Fc γRII, has been considered to be similar to that induced by Fc γRII cross-linking. In this work, we present several lines of evidence to suggest that the mode of platelet activation induced by anti-CD9 mAb is distinct from that induced by Fc γRII cross-linking. Ca2+ release from intracellular Ca2+ stores induced by anti-CD9 mAb depended almost totally upon thromboxane A2 production and released ADP, whereas that induced by Fc γRII was affected only minimally by these factors. Fc γRII cross-linking induced Ca2+ channel opening, which is dependent upon the depletion of intracellular Ca2+ stores. In contrast, anti-CD9 mAb appeared to directly open Ca2+ channels, irrespective of intracellular Ca2+ stores (Kuroda et al., 1995. J. Immunol. 155: 4427). The Ca2+ requirement for the Ca2+ channels opened by Fc γRII cross-linking was also distinct from that induced by anti-CD9 mAb. The early phase of Fc γRII tyrosine phosphorylation was dependent upon thromboxane A2 production with anti-CD9 mAb-induced activation, whereas that of Fc γRII cross-linking was not. p72(syk) and p53/56(lyn) appeared to associate with Fc γRII in platelet activation induced by Fc γRII cross-linking, whereas there was little, if any, association between Fc γRII and these tyrosine kinases in anti-CD9 mAb-induced activation. Piceatannol, a selective inhibitor of p72(syk), enhanced Fc γRII tyrosine phosphorylation induced by Fc γRII cross-linking, whereas it attenuated the process in anti-CD9 mAb-induced platelet activation. It is suggested that the regulatory mechanism of Fc γRII tyrosine phosphorylation differs between these two modes of platelet activation.