The ability of isolated subcellular fractions and cells from the liver of channel catfish Ictalurus punctatus (Rafinesque) to perform key steps in carcinogenesis was studied using the polycyclic aromatic hydrocarbon (PAH) benzo[a]pyrene (BP). Highpressure liquid chromatographic (HPLC) analysis of BP metabolism by catfish liver microsomal fraction revealed the formation of 9,10-diol BP; 4,5-diol BP; 7,8-diol BP; 9-OH BP; 3-OH BP; and quinones. When the microsomal fraction of catfish liver from fish pretreated with 3-methylcholanthrene was used, increased formation of 9,10-diol BP and 7,8-diol BP was seen. Postmitochondrial supernatant (S-9) of catfish liver incubated with required cofactors activated 2-acetylaminofluorene and BP into mutation-causing metabolites in a bacterial mutagen assay. Trypsin digestion of catfish liver was used to isolate 2.8 × 10⁶ cells/g liver with 98 percent viability. Incubation of these cells after 1 day of culture with tritiated BP followed by autoradiographic localization showed preferential accumulation of BP over liver cell nuclei. Subsequent liquid scintillation analysis of cell fractions obtained by cesium chloride centrifugation revealed radioactivity in the deoxyribonucleic acid (DNA) fraction. Results of studies with microsomal fractions demonstrate the capacity of the liver mixed-function oxidase system (MFOS) of this species to metabolize carcinogens into mutagenic forms. Data with isolated cells indicate that activation reactions could result in the initiation of carcinogenesis within the intact liver and suggest the suitability of this species for carcinogen bioassay.