THE CHANGED reactivity of the tuberculous body toward reinfection has been known since Koch's fundamental experiment. A few weeks after the tuberculous infection, allergy, or the hypersensitive state, develops, and the reaction to the tuberculin test becomes positive. Until Rich's1 investigations, allergy and immunity in tuberculosis were thought to have an obligate relation; his studies showed that allergy and immunity could be separated, and allergy abolished and immunity retained (at least under experimental conditions). The degree of the reaction to tuberculin is not, therefore, a measure of the immunity present. Little doubt exists as to the specificity of the tuberculin reaction. Although its action is not clearly understood, the assumption that it is an antigen-antibody reaction is supported by the fact that tuberculous foci and sites of former tuberculin reactions flare up after reinjection of tuberculin. The hypersensitivity of the tuberculous body is not uniformly distributed through all the