Bone marrow stem cell transplant studies carried out by the National Heart Lung & Blood Institute (NHLBI) Bone Marrow Transplantation (BMT) Unit have focused on approaches to optimize the stem cell and lymphocyte dose in order to improve transplant survival and increase the graft-versus-leukemia effect. The aim is to create the transplant conditions that permit rapid donor immune recovery without causing graft-versus-host disease (GVHD) by using reduced post-transplant immunosuppression in conjunction with a transplant depleted of T cells to a fixed low dose, below the threshold known to be associated with GVHD.