1. The human liver UDP-glucuronosyltransferase (UGT) isoforms involved in the glucuronidation of 7-ethyl-10-hydroxycamptothecin (SN-38), the active metabolite of irinotecan (CPT-11), have been studied using microsomes from human liver and insect cells expressing human UGTs (1A1, 1A3, 1A4, 1A6, 1A9, 2B7, 2B15). 2. The glucuronidation of SN-38 was catalysed by UGT1A1, UGT1A3, UGT1A6 and UGT1A9 as well as by liver microsomes. Among these UGT isoforms, UGT1A1 showed the highest activity of SN-38 glucuronidation at both low (1 µM) and high (200 µM) substrate concentrations. The ranking in order of activity at low and high substrate concentrations was UGT1A1 > UGT1A9 > UGT1A6> UGT1A3 and UGT1A1 > UGT1A3 > UGT1A6 ≥ UGT1A9, respectively. 3. The enzyme kinetics of SN-38 glucuronidation were examined by means of Lineweaver-Burk analysis. The activity of the glucuronidation in liver microsomes exhibits a monophasic kinetic pattern, with an apparent K m and V max of 35.9 µM and 134pmol min -1 mg -1 protein, respectively. The UGT isoforms involved in SN-38 glucuronidation could be classified into two types: low- K m types such as UGT1A1 and UGT1A9, and high- K m types such as UGT1A3 and UGT1A6, in terms of affinity toward substrate. UGT1A1 had the highest V max followed by UGT1A3. V max of UGT1A6 and UGT1A9 were approximately 1/9 to 1/12 of that of UGT1A1. 4. The activity of SN-38 glucuronidation by liver microsomes and UGT1A1 was effectively inhibited by bilirubin. Planar and bulky phenols substantially inhibited the SN-38 glucuronidation activity of liver microsomes and UGT1A9, and/or UGT1A6. Although cholic acid derivatives strongly inhibited the activity of SN-38 glucuronidation by UGT1A3, the inhibition profile did not parallel that in liver microsomes. 5. These results demonstrate that at least four UGT1A isoforms are responsible for SN-38 glucuronidation in human livers, and suggest that the role and contribution of each differ substantially.