Total serum bile acid concentrations were studied by an enzymatic-fluorimetric method employing a highly purified 3 alpha-hydroxysteroid dehydrogenase. In 28 control subjects mean total serum bile acid concentration was 2.5 mumoles/1 (S.D. 1.4). In 6 healthy subjects a significant postprandial increase in total serum bile acids occurred with maximal values at 90 and 120 minutes after ingestion of a liquid test meal. The maximal postprandial increase for each subject was 1.5 to 3 times the fasting value. In 7 patients with various hepatobiliary diseases the maximal postprandial elevation of serum bile acids was higher than in the normals, and the duration of serum bile acid elevation was significantly prolonged. In the patients with normal fasting concentration of bile acids the postprandial elevation was also significantly greater than in the controls. A 2-hour postprandial sample seems suitable for the study of the bile acid test meal response for clinical use.