After immunization, normal H-2 heterozygous mice (for example H-2(b) x H-2(d)) generate two populations of cytotoxic effector T cells, one specific for target cells expressing H-2(b)-plus-antigen and the other specific for H- 2(d)-plus-antigen. With a multideterminant antigen, these two populations have about the same activity. We show here that the H-2 type of resident cells in the thymus determines the H-2 preference of cytotoxic T lymphocytes. F(1)(B 10 x B 10.D2) (H-2(b) x H-2 (d)) mice were thymectomized, lethally irradiated, and reconstituted with T-cell-depleted syngeneic hematopoietic cells. Groups of such ATXBM mice were grafted subcutaneously with neonatal thymus lobes from parental mice, either B10 (H-2 (b)) or B10.D2 (H-2(d)). 2-3 mo later, the mice were immunized against the minor histocompatibility antigens on F(1)(BALB/c x BALB.B) cells and assayed for cytotoxic T-cell activity. H-2(b) x H-2(d) ATXBM mice with H-2(b) thymus grafts responded to antigen-plus-H-2(b) much better than to antigen-plus-H-2(d), and vice versa for the mice with H-2(d) thymus grafts. As judged by antiserum treatment, the effector cells were of F(1) origin. To explore the possibility that the "thymus preference" may have been due to suppression of T-cell activity, nonimmune spleen and lymph node cells from normal H-2(b) x H-2(d) mice and cells from H-2(b) x H-2(d) mice bearing a homozygous thymus were mixed 1:1 and immunized in adoptive transfer. The mixture responded to antigen-plus-H-2(b) and antigen-plus-H-2(d) equally well, demonstrating that the cells that showed a "thymus preference" could not suppress a response to antigen in association with the nonthymic H-2 type. We conclude from these and other experiments that H-2 antigens present on resident cells of the thymus determine the spectrum of specificity of T cells which mature in that thymus and eventually make up the peripheral T- cell pool.