To evaluate the environmental impact of different energy technologies, fly ash samples collected from a coal-fired and from an oil-fired electric power plant were used in aerosol inhalation exposures of mice. The effects of multiple 3-h exposures to the fly ash particles at 2 and 1 mg/cu m aerosol mass concentration and <0.5 micrometer MMAD were evaluated in male and female mice by examining the changes in their pulmonary free cells, in their susceptibility to streptococcus infection, and in the bactericidal activity in their lungs to inhaled Klebsiella pneumoniae. Generally, no consistent differences could be discovered in the effects of the exposures between the two sexes. However, in a combined evaluation of both sexes more and greater significant changes relative to controls were observed in the experimental parameters after inhalation of the oil power plant fly ash than after exposure to the coal fly ash. Thus, the overall results of the study indicate that the pulmonary defense system of mice was more adversely affected by the oil-fired power plant fly ash, a true stack emission effluent, than by the coal fly ash collected by electrostatic precipitator, an in-plant control device.