A monoclonal anti-human platelet antibody(MoAb), HIP2 (IgG3) which induced irrevesible aggregation of platelet in association with the release of serotonin and thromboxane B2 formation is described. Indirect immunofluorescence assay (IFS) showed that the antibody binded to platelets and megakryocytes, and gave a weak reaction with aortic, liver and capillary endothelial cells. Electrophoresis of radiolabelled antigen showed that HIP2 recognized platelet membrane glycoprotein lib (130KD). The purified HIP2 MoAb induced aggregation with normal PRP, not with thrombasthenia platelets, formalin-fixed platelets, washed platelets and EDTA-PRP. Washed platelet aggregation with HIP2 could be restored by adding normal plasma or serum into medium, but not by inactivated serum at 56°C for 30 minutes and fibrinogen, The results suggested that the aggregation induced by HIP2 needed Ca++ and complement in medium. HIP2-induced aggregation was completely inhibited by calmodulin inhibitor, compound 48/80, and partially inhibited by aspirin, apyrase, ATP, antimycin A and phentolamine. Anti-glycoprotein Ilia MoAb(SZ-21) inhibited HIP2-induced platelet aggregation, did not block HIP2 binding on platelet surface in IFS. HIP2 no affected platelet adhision on glass. HIP2 prolonged koalin cephalin clotting time in PRP and interruped whole blood retraction. Under electron microscopy, the fibrin formation of clots in the presence of HIP2 was much less than the control. Recently, we found that the platelets from patients with myelo-proligerative diseases had decreased response to HIP2 aggregation. So that, HIP2 MoAb may be a very useful tool not only for study of the platelet membrane structure associated with function of platelet and platelet physiology, but also of the pathogenesis of some platelet diseases.