Radiolabeled preparations of murine surface immunoglobulin have been shown to bind selectively to immunocompetent responder thymus cells. Such bound immunoglobulin (especially IgG) facilitate the mixed lymphocyte culture and cell-mediated lympholysis responses between immunocompetent thymus-cell subsets. Papain digest of IgG also promote alloantigen recognition between immunocompetent thymocytes; most of the activity residues in the Fc piece. The binding of IgG per se does not activate responder thymocytes. Alloantigen-specific MLC and CML responses in vitro appear to require the participation of alloantigen receptors on the responder cell, the expression of alloantigens by the stimulator thymus cells and the binding of B-cell immunoglobulin by the immunocompetent responder thymocytes. When any of these processes are interrupted an alloantigen-specific response is not generated in vitro.