The benzopyrene hydroxylase activity of the methylcholanthrene-inducible form of cytochrome P-450 (P-448) has been studied in native and reconstituted liver microsomal membranes. The data obtained show that the molecular catalytic activity of membrane-bound cytochrome P-448 depends on the molar ratio of the cytochrome to NADPH-cytochrome P-450 reductase and that the optimal ratio for maximal activity of cytochrome P-448 in the microsomal membrane essentially differs from the equimolar one.