Abstract: An anticoagulant principle was obtained from the venom of Agkistrodon halys produced in zhejinang province by means of DEAE-Sephadex A-50 and CM-Sephadex C-25 column chromatography.The effect of the principle on blood coagulation both in vitro and in vivo was studied.In vitro,it remarkably prolonged plasma recalcification time and one-stage plasma prothrombin time,but did not prolong thrombin time and did not dissolve fibrin.It could inhibit formation of blood thromboplastin and tissue thromboplastin even at low concentrations.In rabbit by intravenous injection,the principle caused a marked and transient prolongation of whole blood coagulation time,plasma re-calcification time,one-stage plasma prothrombin time and thrombin time,and decreased suddenly in the rabbit fibrinogen level below 22% at 10 minutes after injection,but Ac-globulin dissolution time was not shortened.