The rate of hemolysis and the behaviour of platelets were investigated during extracorporeal pig liver perfusions. In 8 animals whose iliac vessels had been cannulated and which were perfused for 6 hours in our circuit, packed cell volume, number of red blood cells and concentration of hemoglobin changed moderately after hemodilution with the machine's priming volume. Similiar results were obtained in 10 pigs which were perfused with homologous isolated pig livers up to 6 hours. The best parameter to determine hemolysis was the plasma hemoglobin. Its level was twice as high as the initial value after mixture of the blood with the machine's priming volume in the iliac bypass experiments. Instead it remained practically unchanged throughout 6 hours of extracorporeal liver perfusions. The liver seemed to act as a filter under these experimental conditions. It had similar filtration effects on the enzymes LDH and alpha-HBDG which are set free in hemolysis. The decrease of platelets was more pronounced in extracorporeal liver perfusions than in iliac bypass experiments. Especially the functioning platelets decreased significantly in extracorporeal liver perfusions after 4–5 hours. The filtration effect of the liver seemed to be due to its intact RES. Xenogenous immunreactions played a minor role.