Escherichia coli have been the subject of a vast number of studies since first characterized more than 75 years ago. They constitute the predominant flora of the colon of many animal species, including man, and have been shown to be of great importance in nutrition, in the metabolism of drugs, in certain diarrheal diseases, and in infections of the urinary tract. Conflicting evidence has been presented concerning the role their products play in the pathogenesis of irreversible shock.1-3Under ordinary conditions, these organisms do not appear to disturb the host; in certain circumstances, however, such as during the neonatal period, with hypogammaglobulinemia, localized obstruction, excessive irradiation, adrenal cortical steroid therapy, and neoplasia, they may become invasive and threaten life.4Certain strains which possess hemolytic activity, skin-necrotizing power,5and resistance to the bactericidal activity of serum6,7at times appear to be more invasive than others. In general,