Scooter exhaust particulate matter emissions were found to be mutagenic and to induce potential carcinogenicity. To further explore the mechanisms of mutagenicity and potential carcinogenicity of scooter exhaust particulate matter emissions, immunohistochemistry assays were carried out to detect the expression of some oncogenes and tumor suppressor genes in human diploid cell strain (KMB-13) cells, which were morphologically transformed by the organic extracts of scooter exhaust particulate matter emissions. An ABC diagnostic kit was used to investigate the expression of c-myc, p21, p53, and p16 proteins in the transformed cells. The c-myc and p21 proteins showed marked positive staining compared to control. The data suggest that the mutagenicity and potential carcinogenicity of the scooter exhaust particulate matter emissions correlate with the activation of some cellular oncogenes.