The mutagenic activities of 1-nitropyrene, 2,7-dinitrofluorenone, and a diesel-exhaust extract were compared using the Salmonella typhimurium plate-incorporation assay. Each sample was tested with and without a 9000 x g liver homogenate (S9), both with and without an NADPH-generating system. The samples were also treated with the microsome fraction of S9, cytosol fraction of S9, boiled S9, bovine serum albumin (BSA), and boiled BSA. Salmonella tester strains TA98 and TA98FR1 were used in all treatments; TA98/1,8DNP 6 was to test mutagenic activity without activation. Without the NADPH-generating system, the samples generally had less mutagenic activity than samples treated with the NADPH-generating system. The addition of the NADPH-generating system resulted in marked increases in mutagenic activity of 1-NP in the microsome and S9 treatments, and of all 3 samples in the cytosol fraction treatment. These results indicate that although protein binding reduced the mutagenic activity of diesel-exhaust extract and 1-NP, microsomal activation increased the mutagenic activity of 1-NP. Because 1-NP and 2,7-DNF contributed less than 1.5% of the mutagenic activity of the diesel-exhaust extract, the response to diesel exhaust was not typified by these compounds.