Activation and inhibition phases of fibrinolysis during extracorporeal circulation were studied in six patients perfused by a disc oxygenator and five patients perfused by a membrane oxygenator. The membrane oxygenator appeared to activate plasminogen directly, both in vitro and in vivo, but with the disc oxygenator fibrinolysis was secondary to a hypercoagulable state. Despite activation of fibrinolysis with both types of oxygenator, there were no hemorrhagic complications in any of the patients. It is postulated that the absence of hemorrhagic complications may be related to the normal pre- and postperfusion antiplasmin levels in these patients.