The effects of a 1-hr inhalation of submicrometer sulfuric acid (H2SO4) aerosols via nasal mask on tracheobronchial mucociliary particle clearance and respiratory mechanics were studied in ten subjects with asthmatic histories. A brief inhalation of monodisperse 3.9-micron 99mTc-tagged Fe2O3 aerosol preceded the 1-hr H2SO4 (at 100, 300, and 1000 micrograms/m3) or a sham exposure. Thoracic retention of the gamma-tagged Fe2O3 was measured using external radiation detectors. Respiratory function was measured before, and 15 min and 3 hr after the H2SO4 or sham exposure. After exposure to 1000 micrograms/m3 of H2SO4, the six subjects not on routine medication exhibited a transient slowing of mucociliary clearance and also decrements in sGaw, FEV1/FVC, MMEF, and V25 (P less than 0.05) in both sets of measurements. The four asthmatics on daily medication exhibited stepwise mucociliary clearance that was too variable to allow detection of any H2SO4 effect on clearance. The 1000 micrograms/m3 of H2SO4 did produce decrements in V25 (P less than 0.05), but the variability of the other respiratory parameters in this small group was too great to permit detection of changes. Mucociliary clearance rates in both groups in the sham exposure tests were significantly slower than those of healthy nonsmokers studied previously using the same protocols. The extent of mucociliary clearance slowing following the 1000 micrograms/m3 exposure in the nonmedicated subjects was similar to that in the healthy nonsmokers. This similar change, from a reduced baseline rate of clearance, together with the significant change in respiratory function, indicate that asymptomatic asthmatics may respond to H2SO4 exposures with functional changes of greater potential health significance than do healthy nonsmokers.