1. Pulmonary tuberculosis in unselected rabbits, induced by primary quantitative air-borne infection with human type tubercle bacilli, may retrogress or progress. Some animals whose disease was in a stationary condition might have fallen into one of the above groups had the experiments been prolonged.
2. Within the limits of the observations natural resistance or susceptibility appears to be the chief factor in determining the course of the disease.
3. Following the development of the primary lesions the tuberculin reaction became positive but thereafter proved to be an unreliable indicator of the course of the disease.
4. Tubercle bacilli can be recovered from macroscopically normal lung tissue of rabbits several weeks after primary infection.
5. Reinfection did not induce the formation of new lesions nor alter the course of the disease caused by the primary infection.