The immune response of mouse spleen cell suspension to heterologous erythrocyte antigens and the hapten, trinitrophenol, can be inhibited by the presence of antibody specific for these antigens. The properties of the suppression are: 1) It is antigen specific in that the suppressive antibody must recognize at least one determinant on the antigen. 2) It is not determinant specific in that antibody directed against as few as one determinant on the antigen can suppress the response to the rest of the determinants. 3) The participation of the Fc fragment of the suppressive antibody is indicated by the ineffectiveness of (Fab)′2 and chicken antibody in suppression. 4) The mechanism of the suppression does not appear to be central in that cells preincubated with antigen-antibody complexes for as long as 24 hr are not impaired significantly in their ability to respond to a subsequent challenge with antigen. These findings are discussed in view of the various models which have been proposed for the mechanism of antibody-mediated suppression of the immune response.