By using the in vitro Ames Salmonella/microsomal assay and the in vivo mouse micronu-cleus assay, studies were performed to evaluate the genotoxicity of gasoline exhaust particulate matter generated from five different domestic and imported scooters. In the Ames assay, treatment of test strains TA98 and TA100 with solvent extracts of particulate matter from four of five scooter models caused an increase in the number of histidine-indepen-dent colonies over the background in TA98 without S9 mix. Positive results were also obtained from the micronucleus assay. The frequencies of bone marrow micronucleated polychromatic erythrocytes were significantly higher in the treated compared to the non-treated animals, and the increases in the frequencies were not significantly different among the five types of scooters. Analyses of chemical components showed that scooter exhaust particulate matter contained more than 100 different substances including polycyclic aro-U matic hydrocarbons.