The post-mitochondrial liver fractions (S-9) were prepared from rats and hamsters which have been treated with Aroclor 1254 (PCB) and the capacities of these S-9 fractions to generate mutagenic metabolites from several well known procarcinogens have been compared. Benzo(a)pyrene (B(a)P), 3-methylcholanthrene (3-MC), Aflatoxin B1(AFB1), 2-acetylamino-fluorene(AAF), and 2-aminofluorene (AF) were employed as promutagens in the Salmonella reverse mutation tests. Results showed that the rat and hamster S-9 fractions had differential abilities to produce mutagenic metabolites from a given promutagen.