We studied some aspects of bile acid metabolism in 25 patients affected by Gilbert's syndrome, 5 patients with hemolytic anemia, and 25 control subjects in order to assess whether bile acids as well as bilirubin are affected in unconjugated hyperbilirubinemic conditions. We measured serum cholic and chenodeoxycholic acid conjugates fasting and postprandially, the plasma disappearance of intravenously injected cholyl[1-14C]glycine, 14CO2 in breath, and 14C in stools after oral administration of the same isotope. Mean serum fasting level of conjugated cholic acid was significantly reduced in hyperbilirubinemic patients (p less than 0.01) in comparison with the controls, while the postprandial elevation was similar. The cholyl[1-14C]glycine hepatic uptake was faster in the patients with Gilbert's syndrome, but no significant difference was found as far as 14CO2 in breath and 14C in stools were concerned. Additional in vitro studies showed that increasing bilirubin concentrations displace glycocholic acid and, to a lesser extent, glycochenodeoxycholic acid from their binding to albumin, the affinity constant of the latter bile acid being 30 times greater than that of the former one. This competition between bilirubin and bile acids explains the faster hepatic uptake of cholic acid conjugates and hence their lower serum levels in unconjugated hyperbilirubinemic conditions. In addition, low levels of cholic acid conjugates, together with normal serum chenodeoxycholic acid conjugate levels, discriminate Gilbert's syndrome from other causes of hyperbilirubinemia.