Induction of hepatic microsomal bilirubin UDP-glucuronyltransferase and CYP4A1 P450 by 3,4,3',4'-tetrachlorobiphenyl (TCB) and 3,4,5,3',4'-pentachlorobiphenyl (PenCB) was studied in guinea pigs and rats. Both enzyme activities in guinea pigs were increased 4-fold or more over the control by PenCB treatment, while by treatment with a less toxic congener, TCB, these were increased to a lesser extent. On the contrary, the rat enzymes were significantly decreased by treatment with these co-planar polychlorinated biphenyls. These results suggest that the alterations of both bilirubin UDP-glucuronyltransferase and CYP4A P450 are the characteristic indices for the toxicities of polychlorinated hydrocarbons.