Therapy with unconjugated monoclonal antibodies (mAbs) and radiolabeled mAbs has shown activity in patients with B-cell non-Hodgkin's lymphoma and leukemia. Drug-conjugated mAbs are active in relapsed leukemia. Using these new agents with and after chemotherapy induces a high rate of remission, but this needs to be confirmed in randomized, clinical trials. The antitumor effect of allogeneic stem cell transplantation is being explored through the use of donor lymphocyte infusions for patients who have relapse after transplantation. Attempts to maintain antitumor activity without graft-versus-host disease include CD4 lymphocyte infusions, suicide gene-transfected cells, and the use of cloned T cells more specific for the tumor. Transplantation with nonmyeloablative conditioning regimens relying on the graft-versus-tumor effect of allogeneic lymphocytes has shown preliminary success in the treatment of many hematologic malignancies.