Several experimental approaches, designed specifically to circumvent the possible contribution of a complicating "allogeneic effect," have been successfully used to answer the question of physiologic cooperative interactions between histoincompatible T and B lymphocytes in antibody responses to hapten-protein conjugates. This was accomplished for in vivo cell transfer studies by using an F(1) hybrid host as the recipient of irradiated, carrier-primed T lymphocytes from one parent and 2,4-dinitrophenyl (DNP)-primed B lymphocytes from the opposite strain. Under these conditions, very good T-B cell cooperative interactions were observed to occur between T and B lymphocyte populations derived from syngeneic donors, whereas no cooperative response was obtained when T cells were derived from one parental strain and B cells from the other. Corroborative experiments were performed in a totally in vitro system in which DNP-primed B cells developed good secondary anti-DNP antibody responses in vitro to soluble DNP-keyhole limpet hemocyanin (KLH) when cultured in the presence of irradiated KLH-primed T cells derived from syngenic donors but not from allogeneic donors. The failure of histoincompatible T and B lymphocytes to effect physiologic cooperative interactions has important implications for our understanding of how such interactions normally occur. The possibility that these results reflect the existence of a "block" of some sort to cell-cell interaction by virtue of the presence of a foreign major histocompatibility antigen on the surface of either cell has been definitively ruled out in the present studies. These observations demonstrate that the gene(s) that conditions the capability for physiologic T-B cell cooperation must be shared in common by the respective cell types, and suggest, furthermore, that this gene (or genes) belongs to the major histocompatibility system of the mouse. These findings, together with other relevant phenomena described previously, have led us to postulate that there exists on the B lymphocyte surface an "acceptor" molecule either for the putative active T cell product or for the T cell itself. The important genetic considerations and the possible sequence of events surrounding the actual T-B cell interaction implied by these postulates are discussed in detail.