The possible role of oxygen radicals in mediating the cytopathologic effects of asbestos was studied using human mesothelial cells in culture. Electron paramagnetic resonance measurements of intact cells using the spin trap 5,5-dimethyl-1-pyrroline-1-oxide failed to detect any increase in oxygen radicals in mesothelial cells after exposure to amosite asbestos, although oxygen radicals were readily detected in cells exposed to menadione, an uncoupler of oxidation-reduction reactions. Cellular thiol levels were reduced after exposure to menadione, but were not affected by exposure to asbestos. Addition to the culture media of the free radical scavengers superoxide dismutase, reduced glutathione, N-acetylcysteine, or D-alpha-tocopherol had no affect on the dose-dependent cytotoxicity of amosite fibers. Furthermore, exposure of the mesothelial cells to amosite fibers resulted in no significant increase in the level of DNA single-strand breaks. These results all suggest that for cultured human mesothelial cells, oxygen free radicals are not important mediators of the cytopathic effect of asbestos.