The specificity of two major types of cytochrome P-450 of rat liver microsomes induced by phenobarbital (PB) and 3-methylcholanthrene (MC) toward activation of three 14C-labeled tetrachlorobiphenyl (TCB) isomers to protein-bound metabolites was examined by intact microsomal and reconstituted monooxygenase systems. The monooxygenase system containing PB-inducible form(s) of cytochrome P-450 was much more active than those inducible by MC in activating two TCB isomers, i.e., 2,4,2',5'-[14C]TCB, for the binding reaction. However, the binding of 3,4,3',4'-[14C]TCB was catalyzed most actively by the system containing a MC-inducible (P-448 type) cytochrome P-450 from rat liver microsomes. Thus two forms of cytochrome P-450 had different substrate specificities for the metabolism of TCB. In another series of experiments, the site of protein molecule bound to TCB metabolites was examined by the cytochrome P-448-containing reconstituted system with 3,4,3',4'-[14C]TCB as substrate. The following lines of evidence supported the hypothesis that the active metabolites of TCB bind covalently to cysteine residues of protein molecules a) TCB-binding activity was strongly inhibited by thiol compounds such as cysteine and glutathione and b) of the various kinds of proteins tested as target for TCB metabolites, only those which contained free sulfhydryl groups on the protein molecule were radiolabeled. Moreover, we found that during the metabolism of 3,4,3',4'-TCB, the heme content of the reconstituted system was lowered, probably reflecting the alkylation of the sulfhydryl groups in the active sites of cytochrome P-448.