When lethally irradiated AKR (Mls-1 a ) mice were reconstituted with bone marrow (BM) cells plus a small number (0.5%) of mature T cells from allogeneic B10. AQR or B10 (Mls-1 b ) mice and minor GVHR was induced inthe recipients, almost complete donor chimerism was accomplised in the early stages after reconstitution. By contrast, in irradiated AKR mice reconstituted with T cell-depleted BM cells alone from B10 or B10. AQR mice, radio-resistant T cells of recipient origin persisted for a relatively long period in peripheral lymphoid tissues. In this paper the influence of residual T cells in the chimeric mice on generation of the T cell repertoire derived from donor BM is discussed. It will be demonstrated that the recipient (AKR) T cells are capable of producing Mls-1 a antigens (Ag) after lethal irradiation in vivo. These recipient T cells eventually induce clonal elimination of Mls-1 a reactive Vβ6 + , Vβ8.1 + and Vβ9 + T Cells derived from developing thymocytes of donor BM origin. The Mls-1 a reactive T cells are not eliminated in GVHR chimeras in which recipient T cells are absent. However, Vβ5 + T cells reactive to I-E plus Etc-1 Ag are deleted in the chimeras undergoing GVHR. These results indicate that recipient cells which produce tissue-specific antigens (tolerogens) should be taken into consideration when generation of the T cell repertoire of donor origin following allogeneic BM transplantation is investigated.