Homocytotropic antibody formation against dinitrophenylated Ascaris extracts (DNP-As) was selectively suppressed by the passive administration of homologous antibody against the same antigen. Unrelated antiserum or cross-reacting antibodies against haptenic group or carrier determinants had no suppressing effect. The suppression was preferentially directed to the homocytotropic antibody response, while the hemagglutinating γM antibody formation was unaffected. The effect was most significant when the antibody was administered simultaneously with, or a few days after, the initial antigen injection, whereas previous administration was less effective. The synthesis of homocytotropic antibody was terminated by the passive administration of antibody at the time when the PCA titer was at the maximum. The suppressive activity was detected in the 7S γG fraction of hyperimmune antiserum, while the early 19S or intermediate fractions had little activity. Heterologous antibody to the same antigen showed a similar inhibitory effect, but about twice as much antibody was required for the same degree of suppression. The results indicate the presence of so-called “feed-back” regulation in the homocytotropic antibody response. The possible mechanisms and biologic significances of this regulation are discussed.