Active oxygen species are implicated causally in tumor promotion by 12-O-tetradecanoylphorbol-13-acetate and in cell damage by asbestos, a putative tumor promoter of the respiratory tract. To determine the properties of asbestos important in generation of the oxygen free radical, superoxide (O2-.), hamster and rat alveolar macrophages were exposed in vitro to nontoxic concentrations of fibrous (crocidolite, erionite, Code 100 fiberglass, sepiolite) and nonfibrous (riebeckite, mordenite, glass) particulates. The amount of O2-. released by cells in response to dusts was determined by measuring the reduction of cytochrome c in the presence and absence of superoxide dismutase. Results showed that all fibrous (defined as a greater than 3:1 length:diameter ratio) dusts caused a significant increase in both release of O2-. from rat macrophages and enhancement of zymosan-triggered O2-. from hamster macrophages. Nonfibrous particles were less active than fibers at comparable concentrations. These results suggest that the geometry of particulates is of critical importance in the generation of O2-. from cells of the respiratory tract.