The possible synergistic effects of chronic exposure to coal dust and diesel emissions on various properties of alveolar macrophages were studied in rats. Male Fischer-344-rats were exposed for 7 hours per day, 5 days per week, for 2 years to filtered air, 2 milligrams per cubic meter (mg/m3) coal dust, 2mg/m3 diesel exhaust, or 1mg/m3 coal dust plus 1mg/m3 diesel exhaust. Alveolar macrophages were isolated and assayed for yield, volume, oxygen consumption, chemiluminescence, protein content, lysosomal enzyme activity, and morphology. Exposure to coal dust or diesel emissions had no effect on cellular protein content or membranes. Exposure to diesel exhaust or to the diesel exhaust and coal dust mixture did not affect the number of alveolar macrophages. In contrast, cell yield was increased by 184 percent after a 2 year exposure to coal dust. Exposure to coal dust or diesel exhaust did not alter cell associated lysosomal enzyme activity and did not affect the rate of oxygen consumption of resting alveolar macrophages. Resting chemiluminescence increased in macrophages from rats exposed to coal dust but decreased in those from rats exposed to diesel exhaust or diesel exhaust plus coal dust. Microscopic examination indicated that diesel emissions decreased the ruffling of the cell membrane. The authors conclude that coal dust may activate alveolar macrophages whereas diesel exhaust may depress macrophage activity. A combination of coal dust and exhaust emissions results in an average of the separate responses.