Hepatic microsomal cytochrome P-450 from the untreated coastal marine fish scup, Stenotomus chrysops, was solubilized and resolved into five fractions by ion-exchange chromatography. The major fraction, cytochrome P-450E (Mr = 54,300), was further purified to a specific content of 11.7 nmol heme/mg protein and contained a chromophore absorbing at 447 nm in the CO-ligated, reduced difference spectrum. NH2-terminal sequence analysis of cytochrome P-450E by Edman degradation revealed no homology with any known cytochrome P-450 isozyme in the first nine residues. S. chrysops liver NADPH-cytochrome P-450 reductase, purified 225-fold (Mr = 82,600), had a specific activity of 45-60 U/mg with cytochrome c, contained both FAD and FMN, and was isolated as the one-electron reduced semiquinone. Purified cytochrome P-450E metabolized several substrates including 7-ethoxycoumarin, acetanilide, and benzo[a]pyrene when reconstituted with lipid and hepatic NADPH-cytochrome P-450 reductase from either S. chrysops or rat. The purified, reconstituted monooxygenase system was sensitive to inhibition by 100 microM 7,8-benzoflavone, and analysis of products in reconstitutions with purified rat epoxide hydrolase indicated a preference for oxidation on the benzo-ring of benzo[a]pyrene consistent with the primary features of benzo[a]pyrene metabolism in microsomes. Cytochrome P-450E is identical to the major microsomal aromatic hydrocarbon-inducible cytochrome P-450 by the criteria of molecular weight, optical properties, and catalytic profile. It is suggested that substantial quantities of this aromatic hydrocarbon-inducible isozyme exist in the hepatic microsomes of some untreated S. chrysops. The characterization of this aryl hydrocarbon hydroxylase extends our understanding of the metabolism patterns observed in hepatic microsomes isolated from untreated fish.