The plasma cholic acid, chenodesoxycholic acid and desoxycholic acid levels were studied by spectrofluoremetry in 153 cases. The values of 67 controls with no evidence of hepatobiliary or intestinal disease were compared with those of 86 patients with liver and biliary tract disease. The fasting values failed to provide more diagnostic information than did conventional laboratory assays. Plasma bile acid concentrations exceeding 2.5 mu mol/l are conclusive of liver or biliary disease. A cholic acid/chenodesoxycholic acid quotient higher than 1.0 is a sign of cholestasis. Estimation of bile acids after food intake was found more informative. The plasma cholic acid-and chenodesoxycholic acid levels underwent a considerable increase 1 to 2 hours after meals. A more marked increase of chenodesoxycholic acid than of cholic acid (the ratio of the two being in excess of 1.0) is indicative of cholestasis and is most marked in primary biliary cirrhosis.