The ability of antigen-bearing syngeneic and allogeneic peptone-induced peritoneal exudate macrophages to support development of primary and secondary antibody responses by murine lymphoid or spleen cells in vitro has been investigated. The antigen used was the terpolymer of L-glutamic acid60-L-alanine30-L - tyrosine10 (GAT). Syngeneic and allogeneic macrophages supported development of comparable primary antibody responses to GAT, indicating that genetic restrictions do not limit efficient macrophage-lymphocyte interactions in primary responses. By contrast, immunized spleen or lymphoid cells developed secondary antibody responses preferentially when stimulated in vitro with GAT on macrophages syngeneic to the macrophages used to present GAT during in vivo immunization. Thus, genetic restrictions regulate efficient macrophagelymphocyte interactions in secondary antibody responses. These restrictions have been demonstrated from 2 to 8 wk after a single immunization with limiting quantities of GAT and are controlled by the H-2 gene complex. The implications that immune lymphocytes selectively recognize and respond to antigen presented in the context of the macrophage membrane-antigen complex which sensitized the lymphocytes initially are considered. © 1976, Rockefeller University Press., All rights reserved.