Confirmatory test for Clostridium welchii, consisting of demonstration of the lethicinase activity of the alpha toxin on half of a serum agar plate seeded with the suspect Clostridium. On the other half of the plate the application of Cl. welchii antitoxin neutralizes the toxin as it is formed, and colonies are not surrounded by the opalescent zones signifying lethicinase activity. (The reaction may alternatively be performed in tubes of Fildes’ peptic digest broth with added serum, with and without addition of antitoxin).