The glucuronidation kinetics of 4-methylumbelliferone (4MU) and 1-naphthol (1NP) have been investigated in human liver microsomes to determine the validity of using these compounds as probes for specific UDP-glucuronosyltransferase (GT) activities in human liver. 4MU glucuronidation followed Michaelis-Menten kinetics, whereas 1NP glucuronidation kinetics were biphasic. Cross inhibition studies were performed with 4MU and 1NP to determine the relationship between 4MU glucuronidation and the two phases of 1NP glucuronidation. 4MU glucuronidation was competitively inhibited by 1NP but 4MU inhibited only the high affinity component of 1NP glucuronidation. There was good agreement between the apparent Km values for 4MU and the high affinity component of 1NP glucuronidation and their respective apparent K1 values determined in the cross inhibition studies. These data suggest that the same form(s) of human liver GT is involved in 4MU glucuronidation and the high affinity component of 1NP glucuronidation. A number of compounds known to be specific substrates for purified rat liver GTs were screened for inhibitory effects on 4MU glucuronidation in human liver microsomes. 4-Nitrophenol, 2-aminophenol and androsterone inhibited 4MU glucuronidation whereas bilirubin, chloramphenicol, digitoxigenin monodigitoxoside, morphine, oestrone and testosterone had no effect. 4-Nitrophenol and 2-aminophenol were competitive inhibitors of 4MU glucuronidation but the inhibition of 4MU glucuronidation by androsterone followed atypical kinetics. Overall, the substrate specificity of the human liver 4MU/high affinity 1NP-GT activity appears to be broadly similar to that of the 3-methylcholanthrene inducible rat hepatic microsomal GT.