Summary Foreign sera, their globulin fraction, or washed immune aggregates containing foreign immune globulin were tested for their ability to evoke antibody formation against their immune globulin components. The antibody response against immune globulin was measured by two agglutination methods. In one of these sub-agglutinating amounts of the foreign immune globulin were adsorbed on red cells; in the other the foreign immune globulin against polysaccharide, obtained by dissociation of immune precipitate, was adsorbed to tanned red cells. The data show that guinea pigs injected with normal rabbit serum or its globulin fraction fail to produce appreciable amounts of antibody against rabbit immune globulin. In contrast, injection into guinea pigs of washed immune aggregates formed by the reaction of rabbit immune sera with their corresponding antigens elicits production of potent anti-rabbit immune globulin sera. The ability of washed immune aggregates to evoke this antibody response is inhibited by the admixture of normal rabbit serum to the immunizing antigen. Strong immune sera against the immune globulins of mice and guinea pigs are also readily produced by the injection of immune precipitates containing mouse or guinea pig immune globulin into guinea pigs or rabbits, respectively. Practical and theoretical implications of these findings have been discussed.