1. The in vitro metabolism of indinavir (CRIXIVAN, MK-0639, L-735,524), an HIV protease inhibitor, was evaluated using liver microsomes from cynomolgus monkey, rhesus monkey, chimpanzee and human. Indinavir exhibited marked species differences in metabolism. The overall rate of indinavir metabolism varied > 4-fold among primates (84 pmol/min/mg protein in cynomolgus monkey versus 20.4 pmol/min/mg protein in human) and followed the rank order: cynomolgus monkey > rhesus monkey > chimpanzee > human. 2. The cis-(indan)hydroxylated metabolite of indinavir was formed only in cynomolgus and rhesus monkey livers, whereas trans-(indan)hydroxylation and N-dealkylation were observed as the major metabolites in all primates tested. Inhibition studies with P450-selective inhibitors (ketoconazole, quinine, quinidine) and monoclonal antibodies (against CYP2D6 or CYP3A4) indicated that a cytochrome P450 isoform of the CYP2D subfamily is involved in the formation of the unique cis-(indan) hydroxylated metabolite in monkey, whereas all other oxidative metabolites, including the trans-(indan)hydroxylated metabolite, are formed by CYP3A isoform(s). 3. The present study has demonstrated that monkeys were unique in their abilities to form the stereoselective metabolite and were not appropriate surrogates for the qualitative prediction of indinavir metabolism in human.