Clinical and experimental studies on hemolysis during extracorporeal circulation were performed. Various factors of hemolysis were analized as to perfusion methods, refering to erythrocyte life span. Conclusions are as follows: 1) Concerning hemolysis, ACD blood for priming preserved for 2 to 3 days is not inferior to one preserved for a day. 2) Disc oxygenators give less damage to erythrocytes than bubble oxygenators. 3) Blood from cyanotic heart diseases has a tendency to show more hemolysis than blood from a cyanotic heart diseases. 4) Mechanical hemolysis increases as blood temperature drops. 5) As antihemolytic agents, Mannitol is effective, but Pluronic F-68 shows no recognizable effects in vitro experiments. 6) Blood retension in the pericardium causes marked hemolysis, resulting in elevation of free Hb level in suction circuit, and return of blood from suction circuit to oxygenator gives a rise to elevation of plasma free Hb level in extracorporeal circulation with a little influence to total free Hb level. 7) Hemodilution is effective to decrease damage to erythrocytes. 8) Kind of diluent gives great influence to hemolysis. Gelatin has proved to be best. 9) Hemoglobinuria is seen at higher level of plasma free Hb than 110-140mg/dl after extra-
corporeal circulation. 10) Erythrocyte life span is much shortened with even mild mechanical stimulation to erythrocytes. With strong stimulation, it was markedly shortened.