When the lungs of man or lower animal are removed from the chest under sterile conditions, they contain no bacteria. Yet a person inhales 12,000 liters of air in 24 hours, taking in enormous quantities of particulate matter and bacteria. What is more, when fine emulsions of pathogenic bacteria are injected into the air sacs by way of the trachea, they are retained and destroyed in the lungs without the occurrence of local disease. Stillman,1 for instance, found that "pneumococci which have reached the lungs as a result of inhalation usually disappear within a few hours and give rise to no infection." 12 observed that a dose of pneumococci that kills a rabbit in approximately three days after injection into the blood stream is harmless to an animal when the diplococci have been introduced into the pulmonary parenchyma by way of the trachea. Besredka 3 demonstrated that the