The capacity of four cDNA-expressed human liver UDP-glucuronosyltransferases (UGT), UGT1*6, UGT2B7, UGT2B10 and UGT2B11, to glucuronidate hydroxylated metabolites of benzo[α]pyrene (B[α]P) and 2-acetylaminofluorene (AAF) has been investigated. UGT1*6 and UGT2B7 glucuronidated a range of B[a]P and AAF metabolites with a degree of regiospecificity, although UGTs 2B10 and 2B11 were inactive towards all compounds screened. UGT2B7 glucuronidated the B[a]P trans 4, 5- and 7, 8-dihydrodiols and the 1-, 2-, 4-, 5-, 6-, 8-, 9- and 10-monophenols. In contrast, UGT1*6 lacked activity towards the dihydrodiols and metabolized a more limited range of monophenols, namely 4-, 5-, 8- and 12-hydroxyB[α]P. Both UGT2B7 and UGT1*6 glucuronidated N, 1-, 3- and 8-hydroxyAAF, but 5-hydroxyAAF was metabolised only by UGT1*6. Neither enzyme glucuronidated 3-, 7- and 11-hydroxyB[α]P and 7-and 9-hydroxyAAF, although these compounds were all metabolised by human liver microsomal UGTs. The results suggest that the relative content of UGT isoforms in a cell or tissue will be important for determining the extent to which a particular carcinogen/mutagen is deactivated.