A platelet aggregation inhibitor, named snake venom platelet aggregation dissociator (SV-PAD)-1, with a dissociative reaction of ADP-induced platelet aggregation, was purified from the venom of Protobothrops elegans (Sakishima-habu) by gel-filtration employing Sephadex G-100, and ion-exchange chromatographies using DEAE-Sepharose Fast Flow, CM-Sepharose Fast Flow, and Mono S. By this procedure, about 1.5 mg of purified protein was obtained from 1.0 g of P. elegans venom. The purified protein showed a single protein band and the molecular weight was about 110 kDa on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under reducing conditions. The pI of purified protein showed four-bands of 7.7, 7.8, 7.95, and 8.15. This protein strongly inhibited ADP-induced platelet aggregation in rabbit platelet-rich plasma (PRP), and its IC50 was about 58 nM. It inhibited ristocetin-induced platelet aggregation in rabbit PRP (IC50: 100 nM), but hardly blocked collagen-induced platelet aggregation. This protein promptly dissociated platelet aggregation in rabbit PRP stimulated by high-concentration ADP.