Sensitive, reliable and convenient assays are described for the study of human liver microsomal UDP-glucuronyltransferase. Using 14C-1-naphthol as substrate about 2 mg of a liver biopsy specimen and for 14C-morphine about 20 mg of tissue will suffice for enzyme estimation. Lack of inhibition of 1-naphthol glucuronidation by morphine suggests that the substrates are glucuronidated by different forms of the enzyme. Enzyme levels in the native and activated state were studied in biopsies from patients grouped according to histopathological and clinical criteria. The enzyme assays may help to characterize UDP-glucuronyltransferases in human tissues and their induction by drugs and environmental chemicals, as well as their alteration in various diseases.