Di- ortho polychlorinated biphenyls (PCBs) are prominent environmental contaminants and their biological activity in fish may be more significant than previously thought. Four weeks after intraperitoneal (ip) injection with 50 or 250 mug 2,2',4,4',5,5'-hexachlorobiphenyl (2HxCB)/ g fish, rainbow trout livers were removed and frozen at-80 C or microsomes were prepared. Microsomal ethoxyresorufin O -deethylase (EROD) activity was approximately one and two orders of magnitude greater than controls in fish treated with 50 and 250 mug 2HxCB/g fish, respectively. Cytochrome P4501A (CYP1A) Western immunoblot relative optical density increased with 2HxCB dose. Hepatic CYP1A1 mRNA levels were approximately threefold greater in fish treated with 250 mug 2HxCB/g fish than in controls, while hepatic CYP1A1 mRNA levels in fish treated with 50 mug 2HxCB/g fish were not significantly induced. There was no increase of CYP1A3 mRNA in 2HxCB-treated fish. The study showed 2HxCB induced hepatic EROD activity, CYP1A protein, and CYP1A1 mRNA content in rainbow trout.