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Using the Ames Salmonella-microsome system, we detected mutagenic activity in the exhaust from two kinds of 4-cycle gasoline engines of unregulated and regulated cars, and from diesel engines, as well as in the particulates from air collected in tunnels. The mutagenicity of particulates from a car equipped with a catalyst (regulated car), as compared with that from an unregulated car, was reduced very much (down to 500 from 4500 revertants/plate/m 3 in tester strain TA98). However, the mutagenicity of the ether-soluble acid and neutral fractions from the condensed water of emissions from a regulated car was still high (down to 2880 from 10 900 revertants/plate/m 3 in tester strain TA100). The mutagenic activity of emission exhaust from old diesel car engines was very high; the particulates showed 9140 and 19 600 revertants/plate/m 3 from strain TA98 incubated with an activating rat-liver S9 fraction. A small diesel engine of the type used for the generation of electric power or in farm machinery also produced exhaust with highly mutagenic particulates. The mutagenic activity of a methanol extract of particulate air pollutants collected in a highway tunnel showed 39 revertants/plate/m 3 toward strain TA98 and 87 toward strain TA100. The ether-soluble neutral fraction yielded 86 revertants/plate/m 3 from strain TA98 and 100 from strain TA100. This fraction also contained carcinogenic compounds, including benzo[ a ]pyrene, benzo[ e ]pyrene, benz[ a ]anthracene, benzo[ ghi ]perylene and chrysene. Very high mutagenic activity was detected, especially in the particulate air pollutants collected at night, in another tunnel on a superhighway: 60–88 revertants/plate/m 3 from strain TA100 for the sample collected by day, but 121–238, by night. Night traffic includes many more diesel-powered vehicles compared with gasoline-powered automobiles.