Serum mitochondrial aspartate aminotransferase activity was measured using an immunochemical method in 251 subjects, of whom 140 were chronic alcoholics. The alcoholic patients included 37 with normal liver routine tests (Group I), 61 with noncirrhotic alcoholic liver disease (Group II) and 42 with cirrhosis (Group III), of whom 21 had been abstainers for at least 2 months. All of the remaining 111 subjects were nonalcoholic: 61 had various types of liver disease (Group IV) and 50 were healthy controls. A second assay of serum mitochondrial aspartate aminotransferase activity was performed in 76 alcoholics after a period of abstinence of about 7 days. In addition, serial mitochondrial aspartate aminotransferase determinations were performed in four nonalcoholic volunteers prior to, during and following an alcohol bout. Mean mitochondrial aspartate aminotransferase and mitochondrial aspartate aminotransferase/total aspartate aminotransferase ratio were significantly increased in the alcoholics whatever their liver status, with a sensitivity of the ratio of 81, 85 and 66% for Group I, Group II and the 21 drinkers of Group III, respectively. Only 1 of the 21 cirrhotic abstainers had an increased ratio. Among the 61 nonalcoholic patients with liver disease, 11 had an increased mitochondrial aspartate aminotransferase/total aspartate aminotransferase ratio, specificity of which was 82%. After drinking had been stopped for about 1 week, mitochondrial aspartate aminotransferase decreased by more than 50% and therefore appears as a reliable tool to assess abstinence. In the four cases of alcohol bouts, no significant modifications in mitochondrial aspartate aminotransferase serum values were observed, thus suggesting that mitochondrial aspartate aminotransferase is indeed a marker of chronic, but not of acute, alcohol intake.