SUMMARYThe role of the lymphoid cell in homograft rejection was studied using diffusion chambers placed i.p. The results of these experiments suggest that: (1) chamber-enclosed sensitized lymphoid cells cannot transfer their sensitization to an allogeneic host; (2) chamber-enclosed lymphoid cells in a hyperimmune allogeneic host are destroyed by a humoral substance that is harmless to chamber-enclosed epidermal and thyroid cells; (3) epidermal cells of one strain enclosed with unsensitized lymphoid cells of another strain are not destroyed even if the host, isogeneic with the lymphoid cells, is hyperimmunized. Epidermal cells of one strain (but not thyroid fragments) are destroyed when combined in a chamber with sensitized lymphoid cells of the other strain. Unsensitized lymphoid cells contained in a chamber with a foreign tissue appear incapable of destroying that foreign antigen if isolated from cellular contact with their isogeneic host.