Several in vitro studies have demonstrated the existence of donor derived CD4 and CD8 positive lymphocytes with specific reactivity to recipient leukemia providing a potent graft versus leukemia (GVL) effect, and this GVL effect is area of intense interest both at the NIH and elsewhere. In fact, early attempts to decrease treatment related mortality in chronic myelogenous leukemia (CML) patients undergoing allogeneic BMT by T-cell depletion of the graft resulted in an unacceptably high rate of relapse suggesting that alloreactivity in the donor graft accounted for a significant portion of the cure rate in this disease. This GVL effect is most dramatically demonstrated among relapsed allogeneic bone marrow transplant recipients transplanted for CML in whom a simple infusion of donor lymphocytes can induce a complete and durable remission.