1. A high degree of cellular sensitivity to tuberculin toxicity was demonstrated when explants from tuberculous animals were grown in media containing that substance.
2. Similar degrees of sensitivity were noted in cells derived from animals infected with either virulent or relatively lowly virulent strains of tubercle bacilli.
3. The specificity of the tuberculin cytotoxicity was proven by testing with other bacterial cytotoxic materials.
4. Tuberculin sensitive cells grown in vitro in normal media showed, when tested with tuberculin, persistence of this cellular sensitivity through several transplantations during which time many new generations of cells developed.
5. There was a depression of the initial growth energy of explants from animals during the toxic phase of the disease. During the healing stage the initial growth energy returned to normal although marked sensitivity to tuberculin persisted.
6. The degree of cellular sensitivity to tuberculin in vitro did no parallel the acuity of the infectious process but represented a more or less permanent acquired characteristic impressed on the cell as a result of the infection.