Introduction As the technique of extracorporeal perfusion becomes perfected, the demands made upon it increase. Progressively longer perfusions for more complicated operations upon the heart valves magnify the complications of the perfusion itself. One of the most significant of these complications is postperfusion bleeding. Von Kaulla and Swan25-27 attributed hemorrhagic complications to the association of fibrinolysis and a circulating anticoagulant. Perkins et al15,16 emphasized the role of thrombocytopenia, improper heparin neutralization, and the denaturation of some of the coagulation proteins. Rothney20,21 observed hypofibrinogenemia in one patient perfused for 2½ hours. The lack of unanimity among the various authors is more apparent than real, since several deficiencies play a role in causing postoperative bleeding. In addition, the coagulation factors are so closely interrelated that a change in one factor is seldom a self-limited phenomenon, but often starts a reaction which involves many or all the other factors. The