Aims: To explore the effects in normal and asthmatic adults of exposure to 200 ppb sulphur dioxide (SO2) and 200 mug/(3) and 2000 mug/m(3) aerosols of ammonium bisulphate (AB) and sulphuric acid (SA) (MMD 0.3 mum). Methods: Exposures were placebo controlled, for one hour at rest, double blind in random order. EFEV1 was the primary outcome; secondary outcomes included symptoms, ventilation, exhaled nitric oxide ( NO) concentrations, and nasal lavage fluid ascorbic (AA) and uric acid (UA) concentrations. Results: There were no significant changes in spirometry or symptoms with any exposure in either group. SO2 exposure was associated with an increased respiratory rate relative to air exposure in the asthmatic group (SO2: 958.9 breaths/ hour; air: 906.8 breaths/ hour) but the mean volume breathed did not differ significantly (SO2: 318.8 litres; air: 311.4 litres). AB exposures were associated with a significant rise in [NO] in the asthmatic (+ 1.51 ppb, and + 1.39 ppb), but not in the normal group. Mean pre- and post-exposure [AA] tended to be higher in the normal than in the asthmatic group. Within each group, [ AA] did not change significantly with any exposure. Post-exposure [UA] were greater than pre- exposure concentrations for all exposures, significantly so in the normal group for all exposures except SO2. There were no significant differences in the mean change in [UA] for any exposure relative to air. Conclusions: The pollutant exposure concentrations employed in this study were generally much greater than ambient. It is unlikely that short lived exposures at lower concentrations would show significant effects, but effects of longer term lower concentration exposures cannot be ruled out.