1. The S, R, and Rn variants of the Shiga bacillus are equally toxic. 2. The effect of the toxin upon rabbits is the same, whether it is derived from filtrates of broth cultures (3 to 6 days old), or is obtained by autolysis of the killed bacteria, grown on agar surface. Rabbits show in both cases prostration, loss in weight, paralysis, and diarrhea. 3. When the toxin is heated to 80°C. for 1 hour, its poisonous effect nearly disappears, but its immunizing ability is unaltered. This heated toxin induces a formation of antitoxin, which can protect against the unheated toxins. 4. The anatomical changes observed in the spinal cord (degeneration of the motor neurons) and in the cecum (hyperemia and hemorrhages) are in agreement with the statements of previous authors. Furthermore, the toxin causes hyperemia and hemorrhages in the heart, hyperemia and degeneration in the kidneys and the liver.