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 (O3for 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 mglm3(0.4 ppm). The increased activity observed after exposure to the lower levels of O3is explained as a defensive reaction of the organism. Failure to stimulate activity or a decrease of activity, observed at the higher O3concentrations, may point to adverse events.