Lymphocytes which undergo maturation in the thymus are essential for cellular immunity and for the antibody response to some antigens. In the mouse it has been shown that thymus-derived (T) 1 lymphocytes are required for rejection of skin allografts (1, 2), initiation of graft-versus-host (GVH) reactions (2), development of killer cells specific for histoincompatible tumor cells (3), development of macrophage-mediated cellular immunity (4), initiation of the mixed lymphocyte interaction (MLI) (5), responsiveness to phytohemagglutinin (6), and provision of helper activity in the response to carrier-hapten conjugates and thymic-dependent antigens (7-10).Two distinct subpopulations of thymic lymphocytes exist. The larger, functionally immature population resides primarily in the thymic cortex and is characterized by sensitivity to high dose steroid treatment (11, 12), relatively high cellular buoyant density (13), relatively high representation of the surface alloantigens 0, TL, Ly-A, Ly-B, and Ly-C, and relatively low representation of mouse H-2 histocompatibility antigens (14-18). The smaller functionally mature population resides primarily in the medulla and accounts for about 5~ of the lymphocytes in the thymus (17-19). The mature cells are characterized by steroid resistance, lower cellular buoyant density, absence of the TL surface alloantigen, reduced representation of the 0 and Ly antigens, and increased representation of H-2. The whole of graft-versus-host, mixed lymphocyte interaction, and phytohemagglutinin responsiveness found in the thymus seems to be a function of this smaller mature population (17-21). In addition,