We have already demonstrated that the GPIb binding domain of vWf resided at the regions corresponding to residues 474-488(G10) and 694-708(D5). Moreover, conformational change of vWf was suggested to be important for binding to GPIb. The effect of newly synthetized peptide combining G10 and D5 with lysin(G10-D5) on vWf binding to GPIb and platelet aggregation was studied. All synthetic peptides inhibited both vWf binding to GPIb, ristocetin-induced platelet aggregation and asialo vWf-induced platelet aggregation. G10-D5 possessed the most potent inhibitory activity in the interaction of vWf with GPIb. Only G10-D5 reacted with NMC-4 which recognized the epitope in appropriate conformation of vWf. These results indicate that G10-D5 retains some conformational structure and might be a good tool for anti-thrombotic agent.