Differences in expression of cytochrome P450 forms and their functions in different organs and cell types could determine the response of those cells and organs to xenobiotics. Recently, we described the cellular localization of cytochrome P450IA1 (P450E) induced in 10 organs or organ systems of the fish, Stenotomus chrysops (scup) treated with 3,3',4,4'-tetrachlorobiphenyl or with 2,3,7,8-tetrachlorodibenzofuran. (R.M. Smolowitz, M.E. Hahn, and J.J. Stegeman, Drug Metab. Dispos. 19, 113, 1991). Here we describe the presence and localization of P450IA1 in organs of scup sampled directly from an environment contaminated by chlorinated biphenyls and bibenzofurans, the outer New Bedford Harbor of Massachusetts. Western blot analysis of microsomes from selected organs (liver, kidney, gill, and heart), using monoclonal antibody 1-12-3, revealed induced levels of P450IA1 in each. The localization of P450IA1 in these and other organs was determined in sections prepared by standard histological methods and stained with MAb 1-12-3 in an indirect peroxidase labeling method. P450IA1 was detected in multiple cell types in liver, including hepatic, pancreatic, and vascular tissue. Kidney and gut also showed prominent P450IA1 levels in epithelial structures and in vascular endothelial cells. Specific staining was detected in endothelial cells, but not other cell types, in heart, gill, spleen, testis, ovary, nose, and brain. In heart, the staining was present in the endocardium of atrium and ventricle, and endothelium of the coronary vasculature and great vessels. The results demonstrate that P450IA proteins are induced in many organs of fish exposed to environmental chemicals in the wild, with patterns of cellular localization like those seen in fish experimentally treated with known inducers. The strong staining of P450IA1 in endothelial cells in all organs examined supports experimental results indicating that endothelium is a major site of P450IA1 induction. Our results indicate further that immunohistochemistry is a useful method for detecting P450 induction as a biomarker for exposure.