We have previously shown that the basal uptake of mineral particles by tracheobronchial epithelial cells in organ culture is mediated in part by active oxygen species (AOS) and can be greatly augmented by exposure to cigarette smoke, a concentrated source of AOS, and other radicals. We hypothesized that ozone, another generator of AOS in tissues, might have the same effect. To test this hypothesis, tracheal explants were exposed to room air (control) or ozone in varying concentrations from 0.01 to 1.0 ppm for 10 min, and subsequently to a suspension of either amosite asbestos or titanium dioxide (rutile) for 1 h. Explants were then transferred to an air/CO2 incubator for 1 wk to allow particle uptake to occur, and uptake was determined by morphometry. We found that ozone exposure increased the uptake of both asbestos and titanium dioxide in a dose-response fashion; this effect appeared at lower exposure levels and was more marked with titanium dioxide than with amosite. The ozone effect could be prevented by addition of catalase but not superoxide dismutase to the particle suspension, or by preincubation of the particles with deferoxamine. These observations indicate that ozone can directly increase uptake of mineral particles by tracheobronchial epithelial cells; this effect occurs with brief exposures at very low ozone levels and appears to be mediated by hydrogen peroxide and possibly by hydroxyl radical. These findings support the general hypothesis that AOS are important mediators of epithelial particle uptake in many different settings. Enhanced uptake may be one of the mechanisms by which ozone impairs particle clearance from the lung and may play a role in the increased morbidity seen in populations with exposure to high levels of both ozone and atmospheric particulates.