Aerosol vaccination against tuberculosis is as old as the disease itself. There can be no question that T-irulent tubercle bacilli have been aspirated over the milleniums and have been a great factor in increasing our resistance against virulent tubercle bacilli. The danger, however, of aspirating virulent tubercle bacilli is that there is always the possibility of developing progressive disease; and furthermore, once a virulknt organism lodges in the body, it remains a constant source of danger to the host. It has been shown by Middlebrookl that as few as ten BCG organisms given by the aerosol route will induce in guinea pigs an immunity which is comparable to ten times that dose given subcutaneously or 100,000 times that dose given intradermally. The tuberculin conversion by the various routes of immunization was similar as was the reduction in number of challenge organisms (H37RV) in the lungs and spleens twenty-one days after the challenge as compared to nonvaccinated animals. Similar results demonstrating the superiority of the respiratory route over the percutaneous route have been obtained by Eigelsbach et aL2 in monkeys and guinea pigs with live tularemia vaccine. Aleskandrov et aL38 have obtained evidence of immunologic responses following aerosol vaccination of human subjects (agglutinins, opsono-phagocytic indices, complement fixation, skin test responses) for brucellosis, tuIaremia, anthrax and plague.There are numerous factors which determine the actual number of organisms aspirated int.0 the lower respiratory tract beyond the cilia and especially the terminal broiichioles when subjects are exposed to aerosols. Paramount in this consideration is particle size. Diagram 1 is a schematic representation of the anatomy of the pulmonary tree showing theoretic retention of particles relative to size, according to L a n d~i h l .~ Although there is not universal agreement as to particle size lodging in t,he bronchial tree, it is agreed by most that particles 30 microns in diameter and above are deposited mainly in the trachea. Particles from 1 to 6 microns in diameter reach the respiratory bronchioles and

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