The comparative metabolism of the hydrocarbons, biphenyl and 4-chlorobiphenyl, was investigated using two different preparations of rat hepatic microsomes. The assay was designed to account for all the metabolic products which included the ether soluble lipophilic metabolites, low molecular weight conjugates, and macromolecular adducts, and to determine the effects of induction with Aroclor 1254 and 1248, two commercial polychlorinated biphenyl (PCB) preparations. 4-chlorobiphenyl was the more metabolically active substrate with the induced and control enzymes. In most metabolic fractions biphenyl was less inducible by the PCB's, with the exception of the 2-biphenylol metabolite which was induced ca. 18-fold. Preincubation of the microsomes with carcinogens did not enhance biphenyl 2-hydroxylation. Instead, a general inhibition of metabolic activity was observed for both biphenyl and 4-chlorobiphenyl substrates. Preincubation with phenobarbitone, a noncarcinogen, did not change the microsome-mediated metabolism of biphenyl or 4-chlorobiphenyl. The substitution of a single halogen atom on the biphenyl nucleus altered both the reactivity and pattern of metabolites for these substrates.