The clinical significance of serum bilirubin sulfate, one of the direct bilirubin, was evaluated in various liver diseases with over 2 mg/dl of serum bilirubin concentration. The diagnosis included 25 cases of acute hepatitis, 8 cases of chronic hepatitis, 8 cases of liver cirrhosis and 16 cases of liver cirrhosis with hepatoma. Bilirubin sulfate was fractioned by Yonei's solvent partition method. The clinical significance of bilirubin sulfate was assessed by comparison of bilirubin sulfate with the other bilirubin fractions and with non-specific liver function tests. The results were as follows; 1) The existence of bilirubin sulfate was confirmed in human sera. Bilirubin sulfate concentration correlated with serum total bilirubin and direct bilirubin, and the correlation with direct bilirubin was higher than total bilirubin. 2) The percentage of bilirubin sulfate in total and direct bilirubin increased in the order of acute hepatitis, chronic hepatitis and liver cirrhosis. The percentage of bilirubin sulfate was higher in direct bilirubin than in total bilirubin. 3) Bilirubin sulfate showed the following correlation with non-specific liver function tests; with cholesterol in acute hepatitis, with choline esterase in chronic hepatitis and with GOT, cholesterol and bile acid in liver diseases.