The oxidation of uroporphyrinogen, an intermediate of the heme biosynthetic pathway, by methylcholanthrene-inducible isozyme(s) of cytochrome P-450 has been proposed to play a role in the development of chemically induced uroporphyria. Prior work from this laboratory [lo] indicated that although addition of 3,4,3:4Ltetrachlorobiphenyl is required for uroporphyrinogen oxidation by methylcholanthrene-induced chick embryo liver microsomes, this biphenyl is not required for the oxidation catalyzed by hepatic microsomes from methylcholanthrene-induced rodents. Here we investigated whether rodent microsomes catalyze uroporphyrinogen oxidation without addition of 3,4,3:4Ltetrachlorobiphenyl because the chemical used as an inducer remains bound to cytochrome P-450. Hepatic micro-some8 containing almost no residual inducer were isolated from rats treated with a low dose of 2,3,7,8tetrachlorodibenzo-p-dioxin (TCDD). These microsomes oxidized uroporphyrinogen at high rates without addition of 3,4,3:4'tetrachlorobiphenyl. Inducerfree microsomal cytochrome P-450 was also obtained by inducing cytochrome P-450 in rats and mice with isosafrole, which was then removed from the isolated microsomes by butanol treatment. This procedure resulted in microsomes with high activity for uroporphyrinogen oxidation. Furthermore, addition