The effects of alloantisera against leukocyte alloantigens on plaque-forming cell (PFC) responses to sheep erythrocytes and the terpolymer of L-glutamic acid(60)-L-alanine(30)-L-tyrosine(10) (GAT) by mouse spleen cells in vitro have been investigated. Polyspecific antibodies against both H-2 and non-H-2 alloantigens on responding spleen cells suppressed both IgM and IgG PFC responses; antisera against alloantigens coded for by the K and I regions, but not the D region, of the H-2 complex also effectively suppressed PFC responses. The suppression was not due to cytotoxicity to the spleen cells or anti-immunoglobulin activity in the sera and was directly related to the amount of antiserum added to the cultures. The suppression was specific for spleen cells against which the alloantiserum was directed. The alloantisera suppressed responses most effectively when present during the first 24 h of incubation, and although not rendering lymphoid cells incapable of developing PFC responses after removal of noncell-bound antibody, did act by interfering with successful initiation of the PFC response. The alloantisera suppressed both IgM and IgG PFC responses when directed against alloantigens only on macrophages, but selectively suppressed IgG responses when directed against alloantigens only on lymphoid cells. The alloantisera did not interfere with the ability of macrophages to bind GAT or to support the viability of the lymphoid cells, but did interfere with the ability of macrophage-associated antigen to effectively stimulate antibody responses by the lymphoid cells. Possible mechanisms for the effects of alloantisera on macrophages and the selective suppression of IgG responses when the antisera are directed against alloantigens on lymphoid cells are discussed with reference to our current understanding of genetic restrictions governing cell interactions in the development of antibody responses in mice.