An anticoagulant principle was obtained from the venom of A. halys produced in Zhejiang Province [China] by DEAE-Sephadex A-50 and CM-Sephadex C-25 column chromatography. The effect of the principle on blood coagulation in vitro and in vivo was studied. In vitro, it prolonged plasma recalcification time and 1-stage plasma prothrombin time, but did not prolong thrombin time and did not dissovle fibrin. It inhibited formation of blood thromboplastin and tissue thromboplastin even at low concentrations. By i.v. injection, the principle caused a marked and transient prolongation of whole blood coagulation time, plasma recalcification time, 1-stage plasma prothrombin time and thrombin time and decresased suddently the fibrinogen level below 22% at 10 min after injection in rabbits, but Ac-globulin dissolution time was not shortened.