ABT-737 facilitates mixed chimerism induction. B6 recipients (H-2^b) were treated with a conditioning protocol including TBI (1.5 Gy), MR1 (2 mg), and 25 × 10⁶ fully MHC-mismatched BM cells from CBA donors (H-2^k). Six weeks after BMT, skin transplantation from CBA and BALB/c (3rd party, H-2^d) donors was performed. (A) Adding a short course of ABT-737 to the conditioning regimen (ABT; 50 mg/kg/day, from day −3 to day 12 after BMT) resulted in a higher number of chimeric animals and significantly increased the percentage of donor-derived cells in different cell lineages in peripheral blood leukocytes (PBL; 10 weeks after BMT). CD11b for neutrophils; CD49b for natural killer cells; *P < .05; **P < .01. N = 7. (B) After skin transplantation all chimeric mice accepted donor grafts and promptly rejected 3rd-party grafts, demonstrating that donor-specific tolerance had been induced. N = 7-8 per group. (C) Using the same experimental protocol but without TBI, we obtained a pronounced donor-specific hyporesponsiveness, but tolerance was not achieved, as shown by the slow rejection of donor skin grafts over time. N = 7-8 per group. Representative results of 2 independent experiments are shown. tpl, transplantation.