Tc2-type donor CD8⁺ T cells are enriched in their ability to abrogate marrow graft rejection. Host B6 (H-2^b) mice were irradiated at 950, 650, or 600 cGy; all mice received 1 × 10⁷ TCD bone marrow cells from B6D2F1 (H-2^b/d) donor mice. Engraftment control mice (950/−) and rejection control mice (650/−) received only the donor bone marrow at the time of transplantation; other groups received additional in vitro-generated donor CD8⁺ T cells (1 × 10⁷cells) of Tc1-type (650/Tc1, 600/Tc1) or Tc2-type (650/Tc2, 600/Tc2) or additional unmanipulated donor CD8 cells (650/naive CD8). Each treatment group consisted of 5 mice. Peripheral blood lymphocytes were isolated on day 41 posttransplant and stained with H-2^bFITC (common to both donor and host cells) and H-2^d PE (specific for donor cells); the percentage of donor and host chimerism was then determined by flow cytometry.