The plasma levels of isocitric dehydrogenase (ICD), glutamic-pyruvic transaminase (GPT), glutamic-oxalacetic transaminase (GOT) and glutamic dehydrogenase (GLD) were studied in rats with per oral doses of 0.2ml/100g B. W. of carbon tetrachloride in a 25 % mixture with olive oil and in patients with various liver diseases. Mitochondrial GOT in serum was separated by DEAE-cellulose column chromatography. 1) The elevation in the serum levels of GOT, GPT and ICD from cytoplasma occurred before the onset of parenchymal cell necrosis of the liver in a histologic sense in rats with CCl(4) poisoning. Uniform elevation in the serum levels of the enzymes from both fractions of cytoplasma and mitochondria of liver cell was found in acute icteric period of viral hepatitis as well as in rats in 12 hours to 48 hours with CCl(4) poisoning. In the case of active chronic hepatitis the elevation in serum levels of GOT, GPT and ICD from cytoplasma was predominant as compared with relatively low elevation of mitochondrial GLD. 2) The distinct elevation in serum level of mitochondrial GLD in liver cirrhosis and intrahepatic cholestasis were characteristic as compared with relatively low elevation of the cytoplasmic enzymes in these cases. It is noteworthy that these results reflect sensitively the intracellular lesions in liver diseases. 3) Mitochondrial GOT could be demonstrated only in a remarkable increase of serum GOT levels in the initial stage of acute hepatitis as well as in the exacerbating stage of active chronic hepatitis. On the other hand, in cases of cholestatic liver diseases and cirrhosis no mitochondrial GOT was detected in serum, while another mitochondrial enzyme GLD showed a high plasma level.