Experiments upon dogs anesthetized with nembutal and lasting 4 hours, in which the right lymphatic duct and thoracic duct have been cannulated and collection of lung lymph and blood specimens was accomplished after intratracheal instillation of dog plasma, purified bovine serum albumin, crystallized egg albumin, and hemoglobin, have shown that the absorption of such molecules is slight. Experiments in which pyrex glass spheres averaging 4 micra in diameter were instilled failed to disclose entrance of these distinctive foreign particles into the lymph stream, though the fact that lung phagocytes were often found containing the particles or covered with them, indicated that eventually these particles would be found in lung lymphatics and in lymph nodes. The protection against absorption from the lung alveoli is in the main due to intact alveolar epithelium through which molecules of the dimensions of the proteins. commonly entering the alveoli, as a result of trauma or disease, pass very slowly and are found in small traces in lung lymph and even to a less degree in blood.