Prevention of semiallogeneic bone-marrow rejection mediated by CD4⁺CD25⁺ regulatory T cells is MHC-haplotype specific. B6 mice were lethally irradiated, grafted with a 1:1 mixture of B6 (H-2^b) plus B6D2F1 (H-2^bd) or B6CBAF1 (H-2^bk) bone marrow (5 × 10⁶ cells each), and coinjected with 10⁵ B6 splenocytes and with regulatory T cells from B6 mice preactivated with B6D2F1 (A) or B6CBAF1 (B) APCs in vitro (regulatory-to-effector T-cell ratios as indicated). Bone marrow was analyzed 2 weeks later. Relative percentage of semiallogeneic H-2^bd (□) or H-2^bk (▪) cells in the individual mice is indicated (see “Materials and methods”). ^*P < .05 (Student t test). Combined data from 3 independent experiments are shown. Horizontal bars indicate the mean.