1. Hepatic microsomal enzymes from teleost and elasmobranch fishes catalyse a diversity of monooxygenase reactions, consistent with the presence of multiple, distinct P450 forms. Protein purification and immunological studies have confirmed that multiple microsomal P450s occur in teleosts. 2. A member of the aromatic hydrocarbon-inducible P450 IA family is present in all fish species examined to date. This protein appears to be most closely related to P450 IA1. Certain of the immunological probes for a teleost P450 IA1 (scup P450E) appear to be reagent antibodies, recognizing the homologous protein in members of all vertebrate groups examined. The nature of the epitope recognized by such antibodies is not known. 3. Based on immunological and amino acid sequence comparisons, teleost P450 IA1 appears to be orthologous to both P450 IA1 and P450 IA2 in mammals. Multiple P450 IA genes may appear in teleosts, but divergence on separate lines from that involving mammalian P450 IA2 could include additional, new members (P450 IA3?) of the P450 IA family. 4. There are greater similarities in the N-terminal amino acid sequences of different teleost (scup and trout) P450 IA1 forms, than seen in the N-terminal sequence relationships found in P450 IA1 of mammalian species. Whether this similarity extends to the rest of these teleost proteins is unknown. 5. The induction of P450 IA1 in teleosts involves transcriptional and translational events. However, the temporal patterns involved in induction of mRNA or protein are different from those in mammalian species, indicating additional aspects of the regulation in teleosts. 6. Relationships between other teleost and mammalian P450 forms, or between other P450 forms isolated from different teleosts, remain to be conclusively established. However, certain relationships are suggested, based on catalytic and other comparisons.