A very important role in the pathogenesis of silicosis and berylliosis has been attributed to the interaction of silicone and beryllium with proteins. Since the above compounds reach the organism mainly through the respiratory pathways, it is natural to expect the greatest changes in the proteins of the lung. The objective has been to make a comparative study of the soluble lung proteins in experimental silicosis and berylliosis. Data on the contents of soluble lung protein in experimental silicosis and berylliosis show that the changes occur in opposite directions. While under conditions of experimental silicosis, a sharp decrease takes place in protein concentration when its total content in the tissue is almost unchanged, in berylliosis, on the contrary, an increase in total protein content is observed and a lack of change in protein concentration. With regard to the dry weight of lungs, it characteristically increases both in silicosis and in berylliosis. The increase in the weight of the lungs in rats with experimental berylliosis coincides in time with the marked increase in the total amount of soluble proteins. No such relationship was found in silicosis.