Benzo(a)pyrene [B(a)P] treatment of gilthead seabream, 25 mg/kg, i.p. for 5 consecutive days, did not cause any significant changes in ethylmorphine N-demethylase and aniline 4-hydroxylase activities of liver microsomes. The same treatment did not alter the liver microsomal cytochrome b5 content, NADH-cytochrome b5 reductase and NADPH-cytochrome P450 reductase activities. However, benzo(a)pyrene treatment caused a 2-3-fold increase in 7-ethoxyresorufin O-deethylase (7-EROD) activity of gilthead seabream liver microsomes. Although, upon treatment, total cytochrome P450 content of liver microsomes increased about 1.7-fold in 1990 fall, no such increase was observed in spring 1991. However, a new cytochrome P450 with an apparent M(r) of 58,000 was observed on SDS-PAGE of liver microsomes obtained from benzo(a)pyrene treated gilthead seabream. Besides, in vitro addition of 0.2 x 10(-6) M benzo(a)pyrene to the incubation mixture inhibited 7-ethoxyresorufin O-deethylase activity by 93%. Gilthead seabream liver microsomal 7-ethoxyresorufin O-deethylase activity was characterized with respect to substrate concentration, amount of enzyme, type of buffer used incubation period and temperature.