This paper reports the results of an investigation of the acute effects of air pollution in 321 nonsmoking adults residing in Southern California. Previous epidemiologic investigations of effects of acute exposure to ozone have focused on groups who may not be representative of the general public, such as asthmatics or student nurses. For this study, participants recorded the daily incidence of several respiratory symptoms over a 6-month period between 1978 and 1979. The authors examined the impact of ambient concentrations of ozone, particulate sulfates, and other air pollutants on the incidence of respiratory morbidity, measured as either upper or lower respiratory tract symptoms. Using a logistic regression model, the authors found a significant association between the incidence of lower respiratory tract symptoms and 1-hour daily maximum ozone levels (odds ratio (OR) = 1.22, 95% confidence interval (Cl) 1.11-1.34, for a 10 parts per hundred million (pphm) change), 7-hour average ozone levels (OR = 1.32, 95% Cl 1.14-1.52), and ambient sulfates (OR = 1.30, 95% Cl 1.09-1.54, for a 10-micrograms/m3 change), but no association was found with coefficient of haze, a more general measure of particulates. The existence of a gas stove in the home was also associated with lower respiratory tract symptoms (OR = 1.23, 95% Cl 1.03-1.47). The effects of ozone were greater in the subpopulation without a residential air conditioner. In addition, ozone appears to have had a greater effect among individuals with a preexisting respiratory infection.