This study measured blood coagulation parameters (especially antithrombin III(ATIII) and α2 macrogrobulin (α2 MG) in man to clarify the behavior of blood coagulation and activity corresponding to extracoporeal circulation (ECC). Patients who were operated on under ECC were classified into three groups according to the time of ECC as follows, Group I less 60 minutes, Group II 60 to 120 minutes and Group III more than 120 minutes. Results: 1) Blood coagulation activity returned to the preoperative value within 24 hours after operation in each group. Although AT III had recovered within 24 hours, group III remained at lower levels than groups I and II. AT III rose significantly in patients who received warfarin after prosthetic valve replacement and maintained a higher value than those of untreated group. Consumption of AT III was more in mitral valve replacement than in aortic valve. 2) Plasminogen and α2 macroglobulin fell after operation and required one week to return to the preoperative level and group III showed lower values than the other groups. Fibrin and fibrinogen degradation products (FDP) were elevated longer after operation in group III. These results suggest that prolonged ECC tends to consume much AT III and α2 MG and to cause to rise in FDP, which may lead to thrombosis and DIC. It would be beneficial to give an anticoagulant drug (warfarin) to maintain higher AT III during the early postoperative period when blood coagulation activity recovers.