1. Ultraviolet irradiation of the air of a room exercises a protective influence against natural air-borne contagion of tuberculosis in rabbits.

2. When the radiant energy is of low intensity it reduces considerably the incidence of tuberculosis.

(a) It completely protects rabbits of high natural resistance from acquiring demonstrable disease though they become tuberculin sensitive.

(b) It fails to protect a small proportion of rabbits of low natural resistance from fatal tuberculosis.

3. When the radiant energy is of high intensity all rabbits, whether of high or of low natural resistance, are almost completely protected from a contagion so severe that it is fatal to the great majority of rabbits of the same genetic constitution not protected by these rays. The protected rabbits do not develop tuberculin sensitivity.

4. The contagion of tuberculosis in these studies is air-borne and the radiant energy exercises its protective influence by its bactericidal properties. It is probable that ultraviolet irradiation may control air-borne contagion of human tuberculosis.

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