Tuberculin, in its action as a blood poison for an individual infected with tuberculosis, behaves like many other infectious poisons. Very small fractions of the amount sufficient to threaten life cause a distinct reaction. This is manifested by a rise of temperature preceded by a sharp fall. I know, through personal experience, of a case of human tuberculosis in which more than a hundred times the usual diagnostic dose of tuberculin was administered. But aside from several days of high fever and a considerable feeling of illness, it had no damaging influence on the patient’s general condition. Koch, the discoverer of tuberculin, once took a strong dose of tubercular poison in the form of dead tubercle bacilli and became very ill. In his case probably a hundredth of the amount would have sufficed to cause transient temperature changes and thus have demonstrated that he also had once been infected with tubercle bacilli. Ten years ago I myself reacted to a dose of 4 mg. with fever and a pronounced feeling of illness which confined me to my bed for several days in San Remo. Therefore I have no doubt about the tubercular infection of my body. The most instructive evidence to confirm the general truth of Naegeli’s figures is furnished by the results of investigations made by the Austrian army surgeon, Dr. Franz, on soldiers of two regiments of infantry. In order to avoid injuring the health of the individuals tested, Franz used only very small doses of tuberculin, 1 to 3 mg., which, in case the injection was repeated, was increased to 5 mg. In spite of this, and in spite of the fact that the soldiers represented the healthiest individuals of the population, he found in one regiment in the first year of service (1901) 61%, and in the second year of service 68% of tubercularly infected cases. Franz adds to his report, which at present I have only in manuscript, that when he employed the dose originally recommended by Koch, namely one centigram, his percentage for the twenty-first year of life approached Naegeli’s very closely, 96%! On the other hand the Hungarian investigator Dr. Nikolaus Berend has not obtained a single positive tuberculin reaction in ninety-six very young children, in spite of the fact that among these were some very feeble individuals, and children of parents manifestly tubercular; and further, despite doses as high as 1 cg. We see then that herein also the statistics coincide with Naegeli’s post-mortem statistics.