The toxicity of fly ash from a coal-fired power plant for respiratory tract epithelium was studied in heterotropic tracheal grafts. Hamster tracheal grafts were continuously exposed to beeswax-cholesterol pellets containing 100, 1000 and 5000 micrograms fly ash and evaluated at 1, 2, 4 and 14-15 months of exposure. Histopathologic effects and the autoradiographic pattern of (3H)thymidine incorporation were determined. In all concentrations of fly ash, an early mild submucosal inflammatory response was seen. Morphologic response of the tracheal epithelium was characterized by hyperplasia followed by squamous metaplasia and atrophic lesions. Although a rare papillomatous structure with cellular atypia was seen in grafts receiving 1000 micrograms fly ash, no carcinomas appeared during the 15 month observation period. Varying degrees of submucosal toxicity were also observed during the time period. Autoradiographic studies showed a significant increase in (3H)thymidine incorporation in grafts receiving fly ash treatment.