1. It has been possible to confer on guinea pigs a passive immunity of about 2 weeks' duration to Bacillus welchii toxin through a protective administration of the antitoxin.
2. Guinea pigs which bad received a prophylactic dose of Bacillus welchii antitoxin exhibited pronounced resistance to infection with the virulent bacilli for a period of 12 days.
3. Established infections in guinea pigs with Bacillus welchii have been arrested and controlled by treatment with the antitoxin.
4. The opinion has been expressed that it will be possible to prevent Bacillus welchii infection in man through the prophylactic use of the antitoxin and developed cases of the infection may be controlled by therapeutic injections of the same agent.