The theory that the symptoms of intoxication of many infectious diseases, e. g., cholera, typhoid, plague, dysentery, tuberculosis, are caused by the so-called bacterial endotoxins, that is, substances set free on microbic disintegration as distinguished from toxins which are regarded as true secretions, was advanced by Pfeiffer about sixteen years ago. According to this theory, there is no real secretion of toxins by the bacteria in question, at least not of such kind and amount as to account for the intoxication; on the contrary, the endotoxins are so firmly bound to the bacterial cells that they are liberated only by severe, disintegrative procedures, and in the infected body they are held to be set free only as the bacteria disintegrate and pass into solution under the influence of the antibacterial substances of the blood. The basis for the theory was the observation Pfeiffer made on guinea-pigs injected intraperitoneally with living