The protein C system is a natural anticoagulant and profibrinolytic system consisting of protein C and protein S and thrombomodulin. Because the increased fibrinolysis and the decreased activities of coagulation factor V and VIII are observed in patients undergoing cardiopulmonary bypass, which are known main effects of achivated protein C, we studied the protein C system in the plasma of patients undergoing operation with extracorporeal circulation(ECC group, 31 patients) and without ECC(control group, 10 patients). The nature of the enhanced blood fibrinolytic activity that evolved during extraacorporeal circulation was charaacterized by significantincrease or fibrin degradation product(FDP) in ECC group(P<0.01) but not in control group. The changes of protein C system also showed only in ECC group. The changes of protein C system were most pronounced in the early phase of cardiopulmonary bypass. The changes of FDP ane protein C system were observed in both bubble and membrane oxygenator-used group, but the pattern and degree of change were quite different (P<0.01), i,e., more severe in membrane oxygenator-used group. These results confirm the disturbance of protein C system by extracorporeal circulation, which is possibly activated by the contact activation between blood and synthetic surface. So measurement of changes of protein C system could be used as a good method in the development of new materials for extracorporeal circulation.