In three experiments, the authors have explored the uptake and transport of collidal gold (Au) and iron oxide (Fe2O3) by normal and SO2-injured bronchial epithelium. In the first experiment mice were exposed to a 2-hr aerosol of Au; in the second experiment, mice were exposed to SO2 (250 ppm) for 3 hr, then to Au; and in the third experiment, mice were exposed to SO2 and days postexposure. From each animal, samples of large airways were examined by electron microsopy, and the cell type and particle location were recorded. Au was not readily taken up by the bronchial epithelium in that only two cells out of 1162 contained particles. No Au was seen in the submucosa. Exposure to SO2 produced a mild, but nonnecrotic response characterized by an increase in mucus, vocuolated cells, detached cilia, and an influx of polymorphonuclear leukocytes. SO2 did not affect the uptake of Au; 2 out of 1177 epithelial cells contained Au. However, gold did reach the submucosa. Results from the SO2 plus Fe2O3 experiment were compared to a previous study of similar design in which mice were exposed only to Fe2O3. SO2 increased the uptake of iron and affected the distribution of intracellulat iron among the different cell types. It is possible that increased iron uptake is due to either injury or to changes in particle solubility produced by the SO2.