Koch’s tuberculin is a water-soluble tubercular toxin, given off from the bodies of the tubercle bacilli to the culture medium, and concentrated together with glycerine. Injected either subcutaneously or intravenously it causes no reaction in persons free from tubercular infection. On the other hand, it is one of the strongest poisons for those who are under the influence of such an infection. Even before the infection has led to clearly recognizable lesions, and long before there are any symptoms of tubercular disease, and even if the most careful physical examination fails to discover a suspicion of tuberculosis during the entire lifetime of the individual, his peculiar susceptibility to this tuberculin injection shows that somewhere in his tissues or body fluids tubercle bacilli are producing their peculiar changes. The nature of these changes is becoming somewhat clear to us since there have been discovered in the extra-vascular blood of tubercularly infected men and animals coagulation and agglutination phenomena which are entirely absent in the blood of non-infected individuals. It appears that the activities of the tubercle bacilli in the body of the host excite the production of a soluble anti-body. When this anti-body comes into contact with the water-soluble substances derived from the tubercle bacilli, Koch’s tuberculin, it is transformed into an insoluble body. According to my own researches I believe it probable that this anti-body is formed by the smallest arterioles in the neighborhood of the infected area. The extent of the agglutination phenomena varies according to the amount of anti-body and of the tuberculin with which it comes into contact. This manifests itself clinically, by the degree of fever, and anatomically, by intravascular coagulations. The latter, in some cases may lead to exudations or to the escape of blood from the pathologically altered vessels. As a result of the tubercular poisoning, we would then have, at autopsy, the typical picture of a tuberculin reaction.