In spite of the widespread clinical use of glycyrrhizin, UDP-glucuronosyltransferase (UGT) isoforms involved in the glucuronidation of this drug are still unknown. This report identifies and characterizes the UGT isoforms responsible for glycyrrhetinic acid glucuronidation. In the enzymatic kinetic experiment performed with pooled human liver microsomes (HLMs), Km was 39.4 μmM and Vmax was 609.2 pmol/min/mg protein. Of the baculosomes expressing 12 recombinant UGTs investigated, UGT1A1, 1A3, 2B4 and 2B7 showed catalytic activity and UGT1A3 exhibited the highest activity. Km values of recombinant UGT1A3 and 2B7 were 3.4 and 4.4 mM, respectively. Both imipramine (typical substrate of UGT1A3 and 1A4) and flurbiprofen (typical substrate of UGT2B7) inhibit the glucuronidation of glycyrrhetinic acid. Esti- mated IC50 values were 138 mM for flurbiprofen and 207 μM for imipramine in the inhibition of the glucuronidation of glycyrrhetinic acid in HLMs.