Germ-free and conventionally reared mice of similar genetic stock maintained on the same sterilized diet were compared as to their reaction to bacterial endotoxin. No significant difference was found in the following: elevation in levels of antibody to Gram-negative bacteria, stimulation of glycolytic activity of spleen cells and peritoneal macrophages, susceptibility to infection with Salmonella typhosa, production by endotoxin of increased resistance to infection, and LD50 of endotoxin.