The metabolism of paclitaxel and docetaxel by human liver microsomes was investigated in vitro. The main metabolite of paclitaxel formed in vitro was the 6 alpha-hydroxypaclitaxel: its formation largely exceeded the formation of other metabolites hydroxylated on the lateral chain by rat liver microsomes and initially characterized in rat bile. In contrast, in vitro studied showed that the initial metabolite of docetaxel resulted from the hydroxylation of the tert-butyl of the lateral chain at C13 and that the same metabolites were formed in human and animal models. Comparison of individual CYP protein content of human microsomes and catalytic activities with taxoid biotransformation, showed that 2 distinct isoforms were assigned to the 6 alpha-hydroxylation (CYP2C) and to the hydroxylation of the lateral chain (CYP3A4). Chemical and immunological inhibitions confirmed these assumptions. The effect of antineoplastic drugs potentially associated with taxoids during chemotherapy has been tested in vitro on paclitaxel and docetaxel biotransformations. In the therapeutic range, vincristine, vinblastine, doxorubicine and cisplatin elicited a moderate or no inhibition of paclitaxel and docetaxel metabolism, as well as cimetidine, ranitidine and diphenylhydramine used to prevent major side effects associated with taxoid therapy. In patients given barbiturates, the hydroxylation on the lateral chain of paclitaxel and docetaxel was markedly stimulated and resulted from the induction of CYP3A isoforms. These results clearly demonstrated that the biotransformation of paclitaxel and docetaxel by human liver microsomes was supported by 2 distinct CYP proteins and that drug interactions could modify the therapeutic efficiency of taxoids during chemotherapy.