To investigate the histocompatibility requirements for the macrophage-T cell interaction in the secondary antibody response, splenic T cells from antigen (carrier)-primed F1 hybrid mice were restimulated in vitro with carrier-pulsed F1, parental or allogeneic macrophages. Surviving T cell were cocultured with hapten-primed F1 or parental "B cells" and restimulated with the appropriate hapten-carrier conjugate. The IgG antibody-forming cell response was then measured using a plaque assay. Mapping of the genetic restriction was performed by use of different strain combinations. T helper cells could be restimulated in the presence of macrophages only provided they shared the I-A subregion of the major histocompatibility complex with the F1 T cells frm F1 hybrids restimulated with parental or I-A-identical macrophages were shown to only cooperate with parental B cells of the same I-A haplotype as the macrophages used for restimulation. The defect was at the level of the macrophage, as addition of macrophages of the I-A haplotype used for the restimulation culture reconstituted ability of F1 helper cells to cooperate with the I-A-nonidentical B cells.