One of the most effective ways of preventing lethal graft-versus-host disease (GVHD) after allogeneic bone marrow transplantation (BMT) for leukemia is to remove T-lymphocytes from the transplanted marrow. The reduced early mortality from T cell depletion is however offset by an increased risk of leukemic relapse and infection. We have shown that bone marrow transplants for leukemia depleted of T cells by elutriation and followed by delayed add-back of donor T cells reduces GVHD while preserving an immune response to the hematologic malignancy (the so-called graft-versus-leukemia (GVL) or graft-versus-myeloma effect). The study highlighted a possible benefit of large doses of marrow progenitor cells on transplant outcome. GVHD was reduced but not prevented by T cell depletion of the marrow. The first objective of our BMT studies is to prevent GVHD from the transplant while conserving GVL reactivity. This is a prerequisite to our second objective of determining the risk of GVHD and the benefit from GVL from add-back of donor lymphocytes. These studies will provide the basis for a planned trial adding back donor lymphocytes selected in vitro to confer immunity against infectious agents and residual leukemia without causing GVHD.