The effects of diesel exhaust and coal dust inhalation on rats were investigated. The study design consisted of four exposure regimens: coal dust, diesel exhaust, coal dust plus diesel exhaust, and filtered air (control). Two animal species, Specific Pathogen Free weanling Fisher-344-rats and Cynomolgus-monkeys, were used. Respirable particulate was maintained at 2 milligrams per cubic meter in all exposure groups. Animals were sacrificed and subjected to immunological, biochemical, physiological, microbiological, and pathological evaluations. Extracts of diesel emissions showed marked mutagenic activities in the Ames assay. None of the urine samples from animals exposed to coal dust, diesel emission, or coal dust plus emission showed any mutagenic activity. Sister chromatid exchanges were not detected in peripheral lymphocytes. Cytochrome- P450 enzyme activity in lung microsomes of exposed rats remained unchanged. No evidence of pulmonary function impairment in any group was observed in physiological tests. Light microscopic examination of lungs showed dust deposition with formation of macules in all exposed groups. The authors conclude that there are few pathological effects of significance in, and no evidence of synergism between, diesel exhaust and coal dust.