Cyclosporin G is a new immunosuppressor structurally similar to cyclosporin A. Although this drug is pharmacologically as active as cyclosporin A, it is less toxic, in particular at the kidney level. The aim of this work was to identify the enzyme system(s) involved in the oxidative metabolism of cyclosporin G in man: (1) in a bank of human liver microsomes ( n = 22), cyclosporin G oxidase activity correlated significantly with cyclosporin A oxidase activity ( P < 0.0001) and with the level of CYP3A4 ( P < 0.002), determined by immunoblot; (2) specific inhibitors of CYP3A4, troleandomycin, and ketoconazole, inhibited cyclosporin G oxidase activity by more than 80%; (3) antiCYP3A4 antibodies specifically inhibited this activity by nearly 90%; (4) cyclosporin A was a competitive inhibitor of cyclosporin G oxidase and vice versa; (5). Among a battery of cDNA-expressed CYPs, only CYP3A4 was able to generate detectable amounts of metabolites of cyclosporin G and cyclosporin A with a turnover number close to that calculated from experiments with liver microsomes; (6) in human hepatocytes in culture, pretreatment of cells with rifampicin and phenobarbital, 2 inducers of CYP3A4, produced a great increase in cyclosporin G oxidase activity, while β-naphthoflavone, an inducer of CYPlAs, did not. We conclude that CYP3A4 is the major enzyme involved in the oxidative metabolism of cyclosporin G in human liver.