The effects of polycyclic aromatic hydrocarbons (PAHs) on microsomal liver enzymes were examined in rainbow trout. Various PAHs (naphthalene through benzo(a)pyrene [B(aP]) were injected ip to screen for mixed-function oxidase (MFO) activity. Chrysene, B(a)P, and Aroclor 1254 caused MFO induction. When fish were also exposed to solubilized pyrene, fluoranthene, and B(a)P in water, bioaccumulation of B(a)P resulted in MFO induction, whereas bioaccumulation of pyrene and fluoranthene did not. Based on water and injection exposure to B(a)P, it was predicted that tissue concentrations in excess of 300 μg/kg B(a)P would be accompanied by MFO induction in rainbow trout.