When colon bacilli in sufficient numbers were injected into the peritoneal cavity of rabbits the animals died within a few hours. There was an incubation period of 1 to 1½ hours which was followed by a set of symptoms that culminated in death. The peritoneum, however, showed only very slight changes.

It was thought that the soluble toxic substance of B. coli which was demonstrated by Aronovitch, Zinsser, and Branham may be an important lethal factor. Young cultures of B. coli in broth were centrifugalized and after killing the remaining bacteria by heat the supernatant fluid was injected into the peritoneal cavity of rabbits. There ensued the same set of symptoms terminating in death which were seen on the injection of the whole culture when the supernatant fluid alone was given.

The potency of the toxic substance is variable. This variability is probably in part inherent in the material and in part depends upon the individual resistance of the animals.

Rabbits were injected intravenously with the supernatant fluid of centrifugalized young cultures of B. coli and an antiserum was elaborated against the soluble toxic substance of this organism.

When this antiserum was given intravenously to rabbits immediately or ½ hour after receiving intraperitoneally five times the usual lethal dose of B. coli, ten out of twelve animals survived.

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