In half of the normal mice examined, cultures of mesenteric lymph nodes were positive for enteric bacteria. When a non-pathogenic microorganism, Serratia marcescens, was established in the intestinal tract by administering it to mice in their drinking water, it, too, was recovered from the mesenteric lymph nodes of almost half of the normal mice examined. From these findings it was concluded that bacteria in small numbers were able to pass from the lumen of the unirradiated gut as far as the regional lymph glands. Such bacteria, except the pathogen, Salmonella, were rarely found in liver or spleen, never in the blood of the normal mice. After x-irradiation with 700 r, the incidence of positive cultures showed the liver or spleen became infected with enteric microorganisms before the blood stream was invaded. It appears, therefore, that these elements of the reticulo-endothelial system were able for a time to maintain the sterility of the blood after failure of the more immediate defenses against bacterial invasion from the intestinal tract. It is concluded that if any increased migration of bacteria through the intestinal mucosa resulted from the radiation injury, the increase must have occurred very soon after irradiation.