In this study, the degree of induction of cytochrome P4501A-associated 7-ethoxyresorufin O-deethylase (EROD) activity and immunochemical detection of cytochrome P4501A in leaping mullet (Liza saliens) and common sole (Solea vulgaris) were used as biomarker for assessment of polycyclic aromatic hydrocarbon (PAH) or/and polychlorinated biphenyl (PCB) type organic pollutants along the Izmir Bay on the Aegean sea coast, Turkey. Fish were captured in May 1995 and February and June 1996. Mullet caught from Pasaport, a highly urbanized and industrial section of the Bay, had approximately 62 times more EROD activity than the feral fish sampled from an uncontaminated site in the Outer Bay. Mullet caught along the pollutant gradient at the three other sites in the Bay exhibited less but highly significant induced EROD activity. An inverse relationship was found between the EROD activity in the fish and the distance between the catch point and the discharge region of polluted rivers and of industrial and domestic wastes into the Harbour. Studies using the polyclonal antibodies produced against muller cytochrome P4501A showed a similar trend. In addition, EROD activities of benthic fish, common sole, captured from three different sites of the Bay also confirmed and extended the results obtained with those of mullet. Except that, common sole caught from site 5, the edge of Gediz river, exhibited higher EROD activity than that of fish caught from site 4, Tuzla. Although, detailed qualitative and quantitative analyses of organic chemicals in the waters and sediments of Izmir Bay are not available, the results of this study indicate that Inner and Middle Bays of Izmir Bay are highly contaminated with PAH and/or PCB type organic pollutants. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.