CD9 (p24) antigen exists on the surface of platelet, acute lymphoblastic leukemia cell, eosinophil and other tissues. But the role of this molecule was not certain. One clue is an effect on platelet. Monoclonal antibody that recognizes CD9 (p24) antigen triggers platelet aggregation. We obtained NNKY1-19 that was a monoclonal antibody to CD9 antigen. We used NNKY1-19 to study the functional role of this antigen. NNKY1-19 aggregated human platelets directly with ATP release. NNKY1-19-induced aggregation was associated with a lag time that was prolonged in inverse proportion to antibody concentration. Aspirin had almost no effecton NNKY1-19-induced aggregation. The effect of ONO-3708 (receptor antagonist of TXA2) on platelet aggregation were examined with 1.5mM aspirin in order to inhibit intrinsic TXA2 synthesis. High concentration of ONO-3708 inhibited NNKY1-19-induced aggregation. Our result suggests the possibility that NNKY1-19 target on the platelet membrane is same as that of TXA2. But ONO-3708 had no effect on binding of NNKY1-19 to platelet with flow cytometric analysis. NNKY1-19 may have indirect effect on TXA2 receptor.