1) Pneumoconicosis was induced in rabbits by intratracheal injection of quarts powder and the prophylartic effects of predonisolone on the silicotic lung changes were studied. 2) The lymph nodes and lungs from predonisolone-treated and control groups were histologically examined. In the lymph nodes, there were no significant differences between these two groups, but the control lungs showed proliferation of septal macrophages, thickening of the alveolar wall, scattered quartz powder, and foreign body giant cells. These silicotic changes were seldom seen in the predonisolone-treated lungs. 3) It appeared that the steroid hormone was inhibitory to early silicotic changes and acted to prevent the development of silicosis.