Metabolism of two polychlorinated biphenyls, 3, 4, 3', 4'- and 2, 5, 2', 5'-tetrachlorobiphenyl (TCB), was studied using rat liver microsomes and the four forms of cytochrome P-450 (P-450), P-450b, P-450e, P-450c and P-450d. At first, effects of various inducers of P-450 such as phenobarbital (PB), 3-methylcholanthrene (MC), isosafrole (ISF) and pregnenolone 16α-carbonitrile on the formation of metabolites of these TCBs by liver microsomes were compared. 3, 4, 3', 4'-TCB was significantly metabolized by liver microsomes from MC-treated rats to form two previously reported metabolites, 4-hydroxy-3, 5, 3', 4'-TCB and 5-hydroxy-3, 4, 3', 4'-TCB with a relative ratio of 2.5 : 1. Incubation with microsomes from untreated or PB-treated rats produced none of the metabolites. On the other hand, 2, 5, 2', 5'-TCB was metabolized to 3-hydroxy-2, 5, 2', 5'-TCB most easily by liver microsomes from PB-treated rats and at a moderate rate by liver microsomes from ISF-treated rats. Activities of microsomes from untreated or MC-treated rats to hydroxylate 2, 5, 2', 5'-TCB were low or undetectable. When these TCB hydroxylase activities were examined with a reconstituted system consisting of each P-450, reduced nicotinamide adenine dinucleotide phosphate (NADPH)-cytochrome P-450 reductase, dilauroylphosphatidylcholine and NADPH-generating system, only P-450c catalyzed both the 4-and 5-hydroxylations of 3, 4, 3', 4'-TCB at a ratio of 2.2 : 1. On the contrary, the hydroxylation of 2, 5, 2', 5'-TCB proceeded efficiently with P-450b and P-450e, being more efficient with the former. P-450d did not show any catalytic activity toward 3, 4, 3', 4'-TCB and 2, 5, 2', 5'-TCB. These results indicated that P-450c and P-450b play a major role in the metabolism of 3, 4, 3', 4'-TCB in MC-treated microsomes and of 2, 5, 2', 5'-TCB in PB-treated microsomes, respectively. The inhibition studies using antibodies against P-450c and P-450b further supported the above conclusions.