Graft vs. host response (GVHR)-activated donor T cells bind to stimulatory host cell-derived Ia antigens. Radioimmune cell-binding assays demonstrate that activated donor T cells acquire both host I-A and I-E alloantigens on their surface. Approximately threefold to fivefold less I-E products than I-A products are transferred. Immunoprecipitation and one-dimensional and two-dimensional gel electrophoresis analyses show that radioiodinated alpha and beta polypeptide chains of both I-A and I-E-encoded host Ia molecules may be transferred in an apparently structurally unaltered form from host cells to donor cells. Biosynthetic studies indicate that [35S]methionine-labeled activated donor T cells do not synthesize Ia antigens of the donor haplotype. Functional analyses with fluorescence-activated cell sorter sorted donor T cell subpopulations show that donor T cells that bind host I-A antigens preferentially cooperate with nonimmune host B cells. Donor T cells that do not bind detectable amounts of host I-A antigens preferentially help nonimmune donor B cells. By contrast, donor T cells that either bind or do not bind host I-A antigens display no H-2-restricted interaction and help both donor and host immune B cells. These data reveal that the Ia antigen-binding specificity of distinct functional subpopulations of alloactivated donor T cells regulates their I-region-restricted (self or allo) helper activity for nonimmune B cells but not immune B cells. Furthermore, they suggest that T cell-macrophage and T cell-B cell collaboration is mediated by a complementary anti-Ia:Ia receptor:ligand type of interaction in which the receptor of a T cell binds to the ligand of an antigen-presenting macrophage and/or B cell.