Several in vitro studies have demonstrated the existence of donor-derived CD4 and CD8 positive lymphocytes with specific reactivity for the patient s leukemia. These cells provide a potent graft-versus-leukemia (GVL) effect. This GVL effect is best seen in patients with CML relapsing after BMT, where a single infusion of donor lymphocytes has been shown to induce complete remission. In addition to the potent anti-leukemia effect of these cells, there is now strong evidence that donor T-cells are capable of completely eradicating residual host hematopoietic cells in a non-myeloablative transplant setting (graft-versus-marrow) leading to successful and complete donor hematopoietic engraftment.