Summary: The adjuvant effect on antibody formation of bacterial endotoxins has been shown to be confined to a period of time within several days after antigen administration. Enhancement of the antibody response was demonstrated by injection of endotoxin simultaneously, 6, 12, 24 or 48 hr after antigen. It was not shown by injection of endotoxin 24, 12 or 6 hr before antigen or three to four days after antigen. Initial experiments are described documenting the ability of endotoxin administered simultaneously with antigen to partially restore antibody formation in x-irradiated rabbits.