Five cloned human hepatic UDP-glucuronosyltransferase (UGT) cDNAs were stably expressed in tissue culture cell lines. More than 100 drugs and xenobiotics were used as substrates for glucuronidation catalyzed by the cloned human transferases to determine the chemical structures accepted as substrates. UGT-HP1 exhibited a limited substrate specificity for planar phenolic compounds, whereas UGT-HP4 was more accepting of nonplanar phenols, anthraquinones, flavones, alphatic alcohols, aromatic carboxylic acids, steroids and many drugs of varied structure. UGT-HP3 (bilirubin UGT) catalyzed the glucuronidation of ethinylestradiol. UGT-H6 and UGT-H25 (steroid bile acid UGTs) also catalyzed the glucuronidation of some xenobiotics. Levels of UGT-HP4 activity towards some substrates were sufficient to allow determination of kinetic parameters for the enzyme reaction. Further, metabolism of drugs could be studied by addition to the recombinant cell lines in culture and extraction of the media allowed analysis of glucuronide formation. The protection afforded against cytotoxic drugs was observed. The data presented here demonstrate the potential of using these recombinant cell lines for investigation of phase II metabolism by human UGTs and subtle differences in protein structure which affect their specificity.