Monoclonal antibody 1-12-3 reactive against scup (Stenotomus chrysops) cytochrome P450 E (a teleost CYP IA1) has been used to immunohistochemically localize CYP IA1 within hepatocytes and presumably sinusoidal endothelial and biliary epithelial cells of scup and trout. The goal of the present study was to extend immunohistochemical studies to the ultrastructural level determining intracellular locations of CYP IA1 in fish liver. Juvenile trout (5-10 g) were given i.p. injections once (50 micrograms/g b beta-naphthoflavone in cod liver oil; 0.5-ml injectate volume). After 5 days, livers were fixed (0.25% glutaraldehyde) via vascular in situ perfusion, removed, cut in 100-microns slices, infiltrated, and embedded in LR White monomer. Ultrathin sections of exposed livers were incubated in monoclonal antibody 1-12-3, rabbit anti-mouse IgG, and protein G colloidal gold. Membranes of granular endoplasmic reticulum in perinuclear regions of hepatocytes were consistently labeled. In addition, hepatocyte plasma membrane, particularly microvilli at bile canaliculi, was labeled. Biliary epithelial cells were labeled on luminal plasma membrane surrounding biliary passageway. Plasma membrane facing sinusoid and immediately subjacent cytoplasm was labeled in endothelial cells. Presence of CYP IA1 in sinusoidal endothelium could contribute to detoxication and/or bioactivation of blood borne chemicals. Granular endoplasmic reticulum was not uniformly labeled in hepatocytes. Rather, distribution seemed sequestered within highly specific regions and not dispersed along all membrane surfaces. Localization within biliary epithelial cells could signify potential of this cell type to bioactivate polycyclic aromatic hydrocarbons and may explain the common finding of biliary as well as hepatocytic tumors of trout liver.