The genotoxicity of coal dust and diesel emission particulates (DPs) was studied in rodents. Weanling specific pathogen free Fischer-344- rats and Swiss-Webster-mice were exposed to coal dust, DPs, coal dust plus DPs, or filtered air in inhalation chambers. Exposure was 7 hours a day, 5 days a week, of 2 milligrams per cubic meter respirable dust for up to 2 years for rats and 6 months for mice. Urine samples were collected for 5 consecutive days immediately after the termination of exposure or in the evening between exposures. Particulates from each chamber were extracted with dichloromethane (DCM), evaporated, and redissolved in dimethyl- sulfoxide (DMSO). Urine was passed through a filter column, eluted with DCM, and concentrated. DMSO was added and extracts were concentrated again. Particulate and urine extracts were tested for mutagenic activity with the Ames Salmonella/microsome assay system. Mutations were scored from histidine dependence to histidine independence. Bone marrow cells from 6 month exposed mice were assayed for micronuclei. Blood from 3 and 24 month exposed rats were cultured and assayed for sister chromatid exchanges (SCE). The coal dust extract was not mutagenic with or without S9 activation. Mutagenic activity was observed in DPs and DPs plus coal dust. Mutagenicity was higher in TA-98 than in TA-100 with and without S9 activation. Coal dust plus DPs was much less mutagenic than DPs only. No urine samples from rats exposed to coal dust or DPs for up to 24 months showed any mutagenic activity. Micronuclei mean frequency in polychromatic erythrocytes of mice exposed to coal dust and DPs was twice the control group value, but the difference was not statistically significant. There was no SCE frequency in the blood of any of the rats tested. The authors conclude that animal exposure to DPs or DPs and coal dust by inhalation does not cause genetic changes detectable by SCE or chromosome aberration assay systems.