The in vitro hepatic metabolism of FK 506 was studied in microsomes prepared from control rats as well as in microsomes prepared from rats treated with the selective cytochrome P-450 isozyme inducers 3-methylcholanthrene (IA), phenobarbital (IIB), and dexamethasone (IIIA). The metabolism of FK 506 was similar for control microsomes and for microsomes prepared from phenobarbital and 3-methylcholanthrene induced animals. The percentage of FK 506 metabolized by these tissue preparations ranged from 21.7 to 32.7%. In contrast, the percentage of FK 506 metabolized by dexamethasone induced microsomes was 86.4%. The metabolism of FK 506 was not effected when the selective IA and IIB isozyme inhibitors alpha-naphthoflavone and orphenadrine were added to the incubations. However, the metabolism of FK 506 decreased by approximately 44% when the IIIA specific isozyme inhibitor troleandomycin was added to the dexamethasone induced microsomes. Therefore, the metabolism of FK 506 is apparently mediated primarily by the steroid inducible cytochrome P-450 IIIA isozyme.