A monoclonal antibody (MoAb) H229(IgGl) was obtained by fusion between SP2/0, mouse myeloma cell line and spleen cells from BC3Fl mice immunized with HEL, a erythroleukemia cell line. MoAb H229 precipitated a 55 kilodalton(KD) molecule in reduced condition. It reacted with platelets and megakaryocytes, but not with monocytes, granulocytes, lymphocytes, thymocytes, red blood cells (RBC), colony-forming units-granulocyte/monocyte (CFU-GM), and burst forming units- erythroid (BFU-E). These findings suggest that MoAb H229 reacts with platelets specifically. However, MoAb H229 did not inhibit platelet aggregation induced by ristocetin, ADP, epinephrine and collagen.