The expression of cytochrome P450IA1 was examined in hepatic lesions of mummichog (Fundulus heteroclitus), a small, non-migratory teleost fish collected from a site in the Elizabeth River, VA, heavily contaminated with polycyclic aromatic hydrocarbons (PAH) of creosote origin. Immunoblot ('Western' blot) analysis using monoclonal antibody (MAb 1-12-3) to P450IA1 of the marine fish Stenotomus chrysops indicated that cytochrome P450IA1 levels in hepatocellular carcinoma and in foci of cellular alteration were 28-85% lower than those of adjacent non-neoplastic tissue. P450IA1-dependent monooxygenase activity, measured as ethoxyresorufin O-deethylase (EROD), exhibited a similar trend with EROD activity in lesions being 15-77% lower than activity in non-neoplastic tissue. Immunohistochemical examination of liver sections revealed general low intensity P450IA1-associated staining in hepatocellular carcinoma, exocrine pancreatic tissue, bile ducts and cholangiocellular proliferative lesions. Staining intensity of non-neoplastic hepatic parenchyma varied considerably and was focally distributed. In one case intense staining was observed in an altered hepatocellular focus (putative preneoplastic lesion). The results indicate important similarities in the expression of P450IA1 in neoplasms of fish and mammals and suggest an adaptive response of a wild population to carcinogen exposure.