Ozone effect on antibacterial activity in vivo was investigated by simultaneously determining the physical removal and bactericidal activity rates of the murine lung. Mice were exposed to various concentrations of ozone for 17 hours prior to or 4 hours after infection with aerosols of Staphylococcus aureus labeled with radioactive phosphorus (32P). Bacterial inactivation was expressed as percent bactericidal activity and calculated by comparing numbers of viable bacteria in the lungs four to five hours after bacterial inhalation with numbers present initially minus numbers physically removed. Animals infected and then exposed to ozone showed a progressive decrease in percent pulmonary bactericidal activity which could not be accounted for by physical removal of bacteria. Exposure to ozone prior to infection caused severe ventilatory abnormalities and lesser decreases in bactericidal activity.