The observations recorded in this paper were made in the course of investigations on gas gangrene. The work of McIntosh, of Weinberg and Séguin, and of other investigators has shown that the organisms chiefly responsible for the production of gas gangrene are the bacillus of Welch, the Vibrion septique , and the Bacillus œdematiens . Small amounts of broth cultures of these organisms when injected into animals belonging to a susceptible species, e. g., the mouse or the guinea-pig, produce a violent gas gangrene, and kill the animal within 24 hours. It is known and was confirmed by us that suspensions in saline of B. Welchii and of Vibrion septique from a surface culture are practically non-pathogenic; half a cubic centimetre of a dense suspension of these organisms can be injected into a mouse or guinea-pig subcutaneously or intramuscularly without producing gas gangrene and, indeed, without producing any signs of ill-health. The same result is obtained if broth cultures of B. Welchii or of Vibrion septique , which contain toxins in addition to the bacteria, are centrifuged, and the bacteria, after having been washed free from adherent toxin, are suspended in saline and injected. In the case of B. œdematiens , the toxin is so potent that it is not easy to remove the last traces of toxin by washing, and it is necessary to destroy the last traces of toxin by heating the washed bacteria to 80° C. for half-an-hour, when spores are formed. These spores, when suspended in saline and injected, are again non-pathogenic. Very occasionally it does happen that gas gangrene develops after the injection of detoxicated bacteria or their spores, but such an event is quite exceptional and apparently accidental. With the detoxicated B. Welchii , for instance, we have observed gas gangrene to occur only once out of a large number of experiments on more than a hundred animals. But if these "detoxicated" bacteria or spores are again mixed with a dose of toxin too small to produce an effect by itself, gas gangrene develops regularly and kills the animal.