The addition of heme (1-10 muM) to liver microsomes from phenobarbital (PB)-treated and 3-methylcholanthrene (MC)-treated male rats increased the rate of benzpyrene (BP) hydroxylation by about 20-40%. On the other hand, protoporphyrin IX caused only inhibition of BP hydroxylation. There was no increase of enzymatic activity by heme when solubilized preparations of liver microsomes were used. This suggested the possibility that an apo-cytochrome P-450 was present in intact microsomes. Higher concentrations of heme inhibited BP hydroxylation by either intact or solubilized microsomes. The inhibition by heme with solubilized microsomal preparations was noncompetitive, "mixed-type". However, with intact microsomes, the lack of linearity, precluded the determination of the type of inhibition. To examine possible effects of heme on the binding of BP to microsomal cytochrome P-450, the spectrum elicited by the addition of BP to microsomes was obtained in the presence or absence of added heme. The addition of heme to liver microsomes produced a marked increase in the trough (419-420 nm) of the difference spectrum formed by the subsequent addition of BP. These findings would suggest that heme increased the binding of BP to microsomes. However, the possibility that BP merely displaces the bound heme of the microsomes to yield, as expected, a trough at 413-416 nm (the addition of heme to microsomes yields a peak of 413-416 nm, unpublished) cannot be ruled out. Nevertheless, independent of our understanding of the mechanism involved in the spectral interactions between heme and BP with liver microsomes it is clear that an effect at their binding site(s) must have been elicited by the presence of both compounds.