To test the hypothesis that adaptation protecting against acute effects of ambient ozone (O3) exposures develops in Los Angeles residents, human volunteers were exposed to 0.4 ppm O3 under conditions simulating ambient pollution exposures. Blood biochemical, pulmonary physiological, and clinical responses were assessed. Los Angeles residents (N = 6) showed only minimal clinical or physiological response to O3, while new arrivals (N = 9) showed significant losses in pulmonary function and a tendency toward increased symptoms. Most biochemical responses did not differ significantly between residents and new arrivals. These results agree with others in suggesting that exposures to elevated ambient concentrations of O3 produce adaptation in a least some residents of photochemical pollution areas. The underlying mechanisms and long-term consequences of such adaptation are unknown.