A three-year research study was conducted investigating the effects of individual and sequential exposures to ozone and sulfuric acid aerosol on pulmonary function and bronchial reactivity in human subjects. PHASE I: In healthy smokers and nonsmokers exposed for 4 hours to 98 micrograms/cu. m 0.14 micrometers H2SO4 aerosol, no significant changes in pulmonary function (FVC, FEV1, FEV3, SGaw, FRC, SBNER, Phase IV, CL stat, CL dyn, RL) were observed with exposure or 24 hours post-exposure. PHASE II: The effect of 0.3 ppm O3 on a subsequent exposure to 100 micrograms/cu. m, 0.13 micrometers H2SO4 aerosol was studied in nonsmoking subjects to determine if pre-exposure to O3 would sensitize these individuals to H2SO4 aerosol. Two-hour O3 and 4-hour H2SO4 exposures were empolyed. Pulmonary function (FVC, FEV1, FEV3, SGaw, FRC) and bronchial reactivity to methacholine were measured following single and sequential exposures. No significant changes in pulmonary function were observed with the exposures. No significant increase in bronchial reactivity occurred with any exposure, although a nearly significant decrease resulted following the single H2SO4 exposure. The authors conclude no decrement in function occurs with exposure to respirable H2SO4 aerosol individually or when preceded by O.3 ppm O3.