CYP functions may determine an organism's susceptibility to toxic chemicals. Most fish CYP1As have been classified as CYP1A1 (Nelson et al. (1996) Pharmacogenetics 6, 1-42) but may be like ancestral genes that diverged to CYP1A1 and CYP1A2 in mammals. This might be evident in catalytic function of fish CYP1As. To test this hypothesis, an expression plasmid bearing a scup CYP1A cDNA was transfected into Chinese hamster V79 cells, which then expressed authentic and active scup CYP1A. These V79 cells were exposed to phenanthrene (Ph), chrysene (Ch), benzo[c]phenanthrene (B[c]Ph) or benzo(a)pyrene (BaP) and metabolites analyzed by HPLC and GC-MS. Sites of attack by the expressed scup CYP1A were 1,2- and 3,4-Ph, 1,2- and 3,4-Ch, and 3,4- and 5,6-BcP. Comparing these patterns to those formed by human and rat CYP1A1 or 1A2 expressed in V79 cells showed that Ph and B(c)Ph metabolism by scup CYP1A1 resembled that of human 1A2, Ch metabolism resembled that of human 1A1 and rat 1A1. As with induced scup liver microsomes, purified and reconstituted scup CYP1A and antibody inhibition studies, expressed scup CYP1A oxidized BaP at the 7,8- and the 9,10-positions more efficiently than mammalian 1A1 or 1A2. Activation of polycyclic aromatic hydrocarbon (PAH) and PAH dihydrodiols to cytotoxic derivatives by expressed scup CYP1A also showed similarities to CYP1A1 or CYP1A2. Thus, scup CYP1A1 has catalytic properties resembling both CYP1A1 and CYP1A2 from mammals, consistent with the possibility that teleost CYP1As represent a type ancestral to both mammalian 1A forms. The metabolite and bioactivation patterns are consistent with fish CYP1As being able to activate PAH promutagens efficiently, with possible toxicological significance. Metabolite profiles are being used to model the active site of scup CYP1A.