1) The effects of secondary injections of various soluble protein antigens, subsequent to disappearance from the circulating system of antibodies to a primary antigen, are described. 2) Secondary antigen injections do not elicit antibody responses to previously injected antigens, if the antigens are unrelated. 3) Quantitative levels of antibody response to secondary antigens are apparently unaffected by previous antigenic stimulations of the animal, if the antigens are unrelated.