Mycophenolic acid, the active metabolite of the immunosuppressant and antiproliferative agent, mycophenolate mofetil, is primarily metabolized by glucuronidation to the inactive 7-O-glucuronide. Although the uridine diphosphate (UDP) 7-O-glucuronide is the principal excretion product of this drug, carboxyl-linked glucuronides have also been detected in vitro and in vivo. To identify human UDP glucuronosyltransferases that are active in the glucuronidation of mycophenolic acid, cDNAs encoding individual UDP glucuronosyltransferase forms have been expressed in cell culture, and the capacity of the expressed enzymes to use mycophenolic acid as a substrate has been assessed. Two UDP glucuronosyltransferase forms, UGT1A8 and UGT1A10, were active in the glucuronidation of mycophenolic acid. Both enzymes are predominantly expressed in the gastrointestinal tract and hence, may play a role in the metabolism of mycophenolic acid in the gastrointestinal tract and in the acquisition of resistance to the mito-inhibitory effects of this drug in cultured human colorectal carcinoma cell lines. The identities of the UDP glucuronosyltransferase forms that are mainly responsible for the glucuronidation of mycophenolic acid in the liver and kidney remain unknown; however, UGT1A9 may be important in this respect as the cDNA-expressed enzyme has some capacity to glucuronidate mycophenolic acid. Other UGT1A forms in the liver and kidney (UGT1A1, UGT1A3, UGT1A4, and UGT1A6) were inactive toward mycophenolic acid.