The effect of specific anti-I-J reagents on macrophage-T cell interactions was studied in an in vitro antibody response to burro erythrocytes. Macrophages were prepared from the spleens of F1 hybrid mice whose parental strains differed at the I-J subregion. Two F1 hydrids were used for these experiments, [B10.A(3R) x B10.A(5R)]F1 and [B10.s(9R) x B10.HTT[F1. F1 macrophages responded equally well with F1 T-B cells or with T-B cells of either parenteral strain. When F1 macrophages were pretreated with anti-I-J serum (without complement) specific for one parenteral haplotype, they were only able to cooperate with T helper (Th) cells of the unblocked haplotype and with F1 Th cells. Identical results were obtained with (J(b)xJ(k))F1 and (J(s)xJ(k))F1 mice. The results indicate that Th cells possess genetically restricted receptors for macrophage I-J-subregion gene products and that the interaction between this receptor and the macrophage I-J subregion determinant is essential for the initiation of a primary in vitro antibody response to an erythrocyte antigen.