Dependable immunity is established against smallpox by vaccination—why not against tuberculosis? This is the question Koch asked in 1882, and it is still being asked in 1950. Literally scores of physicians and scientists have diligently sought the answer. No pathogenic micro-organism has deceived the medical profession with respect to immunity as often, as continuously or as completely as the tubercle bacillus. HISTORICAL AND EXPERIMENTAL BACKGROUND In 1920 and 1921 I was assigned to projects that offered unlimited opportunities to study tuberculosis in all of its stages of development. One of them was concerned with aiding in the organization and direction of a tuberculosis control program among students of the University of Minnesota; the other, with a tuberculosis control program for all school children of Minneapolis. I also participated in other projects that included persons beyond university age and even through the period of senility. The literature revealed numerous attempts to