Macrophages obtained by lavage from the lungs of rats exposed to 0.1 or 0.2 mg/m3 (0.05 or 0.10 ppm) ozone (O3) for 16 h show increased activity as measured in vitro by cell adherence to nylon fiber. A difference in adherence is noticed between macrophages occurring in first or second lung washings. The latter are less sensitive to ozone stimulation. At ozone doses higher than 0.2 mg/m3, the effect disappears and is even negative for second-washing macrophages after exposure of rats to 0.8 mg/m3 (0.4 ppm). The increased activity observed after exposure to the lower levels of O3 is explained as a defensive reaction of the organism. Failure to stimulate activity or a decrease of activity, observed at the higher O3 concentrations, may point to adverse events.