This study was intended to help explain individual differences in susceptibility to irritant effects of ozone (O3), by determining whether prior ambient O3 exposures and/or recent acute respiratory illness modified response to laboratory O3 exposures. Response was measured in terms of lung function changes and irritant symptoms. Initially, 59 adult volunteer Los Angeles area residents underwent screening exposures in spring, before the season of frequent high ambient O3 levels. Unusually responsive and nonresponsive individuals (N = 12 and 13 respectively) underwent followup exposures in autumn (late in the high-O3 season) and in winter (low-O3 season). All exposures were to 0.18 ppm O3 for 2 hr with intermittent heavy exercise at 31 degrees C and 35% relative humidity. Nonresponders tended to remain nonresponsive throughout. In fall, responders had lost much of their reactivity, as if they had "adapted" to summer ambient O3 exposures. They did not regain reactivity by winter. Clinical laboratory findings suggestive of acute respiratory illness did not appear to correlate with O3 response. Eight responders and 9 nonresponders underwent another followup exposure in spring, about 1 yr after screening. By that time most responders had regained their reactivity; individual function changes were significantly correlated with changes 1 yr earlier. These results suggest that response to O3 is a persistent individual characteristic, but can be modified by repeated ambient exposures.