A newly developed enzymatic method for determining urinary 3 alpha-sulfated bile acids was used to measure serum 3 alpha-sulfated bile acid levels in 114 patients with hepatobiliary diseases and 56 healthy subjects. The lowest measurable amount of the 3 alpha-sulfated bile acids was 0.5 mumol/liter. The standard curves for glycolithocholic acid 3 alpha-sulfate, glycoursodeoxycholic acid 3 alpha-sulfate, and lithocholic acid 3 alpha-sulfate were linear from 0.5 to 250 mumol/liter. Specificity of the assay was satisfactory and intra- and interassay variations ranged from 0.8 to 4.4% and from 1.2 to 7.9%, respectively. Analytical recovery was more than 91%. The values obtained by this assay were well correlated with those by gas-liquid chromatography measurement (r = 0.91, P < 0.01). The fasting serum 3 alpha-sulfated bile acids level in healthy subjects ranged from undetectable to 1.9 mumol/liter (mean +/- SE; 0.9 +/- 0.1 mumol/liter). The percentage of 3 alpha-sulfated bile acids in total bile acids (sum of 3 alpha-sulfated and 3 alpha-hydroxy bile acids) in serum was 16.8 +/- 1.5%. In subjects with hepatobiliary diseases, serum 3 alpha-sulfated bile acids levels were elevated; however, the percentage of 3 alpha-sulfated bile acids in total bile acids was decreased and correlated with the severity of hepatocellular insufficiency. This enzymatic assay is simple, rapid, and accurate for the determination of serum 3 alpha-sulfated bile acids.