Monoclonal antibody 1-12-3 (MAb 1-12-3) recognizes an epitope exclusive to cytochrome P450s in subfamily 1A (CYP1A) from all vertebrates tested so far, including one amphibian species. In this study, we first tested the utility of MAb 1-12-3 for detection of presumed CYP1A proteins in hepatic microsomes of northern leopard frogs treated without or with 3,3',4,4',5-pentachlorobiphenyl (PCB 126). Statistical analysis showed that ethoxyresorufin-O-deethylase (EROD) activities and CYP1A equivalents in treated groups were significantly increased at doses > or = 2.3 mg/kg compared with the control groups (p < 0.05), and the increases were maintained for at least four weeks. This result confirmed that MAb 1-12-3 can be used for detection of CYP1A in northern leopard frogs and indicated that CYP1A is the primary catalyst for EROD in this species. In a subsequent experiment, sections of organs of PCB 126-treated frogs were immunohistochemically stained with MAb 1-12-3 to identify localization of the CYP1A in different cell types. The CYP1A staining was seen prominently in hepatocytes and epithelium of nephronic duct, while capillaries close to gastric epithelium and submucosal vascular epithelium in both stomach and intestine exhibited moderate to strong staining. The CYP1A was immunodetected in coronary endothelium and the vascular endothelium of lung and gonad. In skin, mild staining was seen in epithelial cells of mucous glands and serous glands and in vascular endothelium, demonstrating induction of CYP1A in the dermal layer.