The role of metabolic activation in the difference between a microbial and mammalian bioassays in the ranking of genotoxic potency of several environmental emissions was investigated. Although the relative potency in the Ames assay correlated well with the relative potency in mammalian cell and mouse skin for a series of automotive emissions (diesel and gasoline) this was not the case for the coke oven, roofing tar and cigarette smoke condensate (CSC) emissions. The study examined several parameters of the metabolic activation with Salmonella typhimurium TA98 including S9 concentration and a comparison of Aroclor-1254 induced with uninduced S9 from both rat and hamster liver. The diesel emissions sample was direct acting while the other samples required activation. The standard S9 concentration (approximately 1.25 mg protein/plate) also produced the maximum mutagenic activity. Induced S9s produced higher mutagenic activity than uninduced. The hamster S9 gave significantly higher mutagenic activies than rat S9 for the coke oven and CSC.