IDENTIFICATIONs of bacterial origin elaborated in strangulation intestinal obstruction has been a major project of this laboratory.1Clostridial exotoxins have been implicated as lethal agents in experimental studies with rabbits,2guinea pigs (unpublished data), rats,3and pups.3Other investigators have implicated bacterial endotoxins4and viable bacteria5as the toxic agents. Terminal peritoneal fluid and loop contents have been the specimens usually examined for toxicity. The bacterial population of peritoneal fluid aspirated at intervals from the same dog has been shown to remain low until 12 to 18 hr after surgery, when the phase of logarithmic growth was achieved by the endogenous intestinal bacteria.6In the work reported here groups of dogs were sacrificed at specified times after strangulation to obtain the contents of isolated closed loops. Methods The intestines of 28 dogs were divided at a site three fourths of the distance