Lethally irradiated conventional mice were killed at daily intervals after irradiation. A high correlation was found between the occurrence of bacterial overgrowth in the digestive tract and invasion into the regional lymph nodes the spleen and the blood. By oral contamination of mice with quite high doses of either an exogenous or an endogenous biotype of an Enterobacteriaceae species at day 4 after irradiation, it was attempted to induced such condition of intestinal overgrowth. In all animals such an abnormal colonization of the contaminant was indeed achieved in this way and the mice died from an infection caused by the biotype used for the contamination. The interval between contamination and death was different when different biotypes were used for contamination and occurred sooner after irradiation when an endogenous biotype was used for (re)contamination. It is concluded that oral contamination with exogenous as well as with endogenous potentially pathogenic bacteria during a period of severely decreased resistance to infections must be avoided. The colonization resistance of the digestive tract of these animals should be maintained as high as possible.