Lysis of fibrin was first recognized by MORGAGNI in 1769, observing a liquid blood in a patient of acute death, and the phenomenon was named as fibrinolysis by DASTRE in 1893. In 1937, MACFARLANE recognized in a patient after cholecystectomy that the blood clot was lysed completely in the following morning. Since then, much attention has been paid clinically on fibrinolysis and it has been said to occur in case receiving a large amount of blood transfusion, shock, cancer, obstetric diseases, hemophilia, various drug poisonings, allergic diseases, after irradiation and after the operations of lung, pancreas and prostate. In our department, also, the similar phenomenon was recognized often in association with cardiac surgery using the artificial heart-lung machine, and a difficulty in hemostasis was encountered postoperatively. We have been studying, therefore, on fibrinolysis in open heart surgery.