Microsomal UDP-glucuronosyltransferase activity toward the bile acids (chenodeoxycholic, deoxycholic, ursodeoxycholic, lithocholic, and glycolithocholic) has been detected in human specimens of liver, kidney, and intestinal mucosa. The characteristics of hepatic and extrahepatic UDP-glucuronosyltransferase activities toward these bile acids were compared with respect to kinetic parameters and other catalytic properties. Whereas no organ-specific differences in the affinities of individual bile acids to hepatic and extrahepatic UDP-glucuronosyltransferases were observed, the individual bile acids showed reaction rates in liver that were about twice the rates estimated in kidney and about twice to three times the rates observed in duodenal mucosa. In intestinal mucosa the rate of chenodeoxycholic acid glucuronidation exhibited a progressive decrease from duodenum to colon, where it was 30% of the duodenal level. Comparison of the glucuronidation rates that were estimated with different bile acids in hepatic or extrahepatic tissues showed that for each organ a bile acid structure-activity relationship existed, with highest activity observed for lithocholic and ursodeoxycholic acids, which was about twofold higher compared with chenodeoxycholic or deoxycholic acids. Lowest activity was estimated for glycolithocholic acid. UDP-glucuronosyltransferase activity toward chenodeoxycholic acid was studied in biopsy specimens of liver that were obtained from a large group of patients with the following liver diseases: liver cirrhosis, liver fibrosis, granulomatous hepatitis, fatty liver hepatitis, and fatty liver. A significant decrease in enzyme activity was observed in patients with liver cirrhosis and in patients with granulomatous hepatitis compared with patients without liver disease.