Data are presented showing that endotoxins absorbed from the intestine are present in the blood, and in much higher incidence in portal than in systemic vein blood. The incidence of endotoxemia in portal vein blood varies with the size of the intraintestinal pool of endotoxin. A reduced capacity of the R.E. system in the liver for extracting endotoxin allows endotoxin in the portal vein blood to traverse the liver and enter the general circulation.