In this study a possibility was examined that the specific antigen might inhibit antibody forming cells in order to know whether a non-T-cell-dependent mechanism of self-tolerance is present or not. If an autoantigen inhibits autoantibody forming cells, this inhibition may be one of the non-T-cell-dependent mechanisms of self-tolerance. Spleen cells from mice which were immunized with sheep red blood cells (SRBC) were incubated with SRBC and after an hour incubation erythrocytes added were removed. After 4 and 8 hr incubation, the plaques against SRBC were assayed by the plaque method. 1) After 8 hr incubation, SRBC reduced the plaque count to about 60% of the control aliquot incubating spleen cells with C3H mouse red blood cells. 2) This inhibition required 1% SRBC suspension, but despite increase in its concentration to more than 1% the degree of the inhibition remained constant. 3) This inhibition seemed to require neither T cells, macrophages nor the cells which display the cytotoxic activity in the antibody dependent cell-mediated cytotoxicity.