Allogeneic bone marrow transplantation (BMT) has been associated with an antileukemic effect, the graft-versus-leukemia (GVL) reactivity. Since T-cell depletion of the bone marrow graft performed to reduce the incidence and severity of graft-versus-host disease (GVHD) after BMT has been associated with an increase risk of relapse, the GVL reactivity has been attributed to the T lymphocytes from the graft. Previously, we demonstrated that leukemia-reactive cytotoxic T-lymphocyte (CTL) lines and clones could be generated from the peripheral blood of HLA-genotypically identical siblings of patients with leukemia by stimulation of the donor cells with irradiated leukemic cells from the patients. HLA class I as well as class II restricted CTL clones could be generated that recognized the leukemic cells. Some clones recognized the leukemic cells from the patient, but not the interleukin (IL)-2- stimulated lymphocytes from the same individual. To explore the possibility of clinically using donor-derived CTL lines directed against the leukemic cells from patients who relapsed after allogeneic BMT, a pilot study was performed using eight donor-recipient combinations. In seven of eight combinations donor- derived CTL lines could be generated that showed specific lysis of the leukemic cells from the patient. In five of these cases, the CTL lines showed reactivity with the leukemic cells, but not with the IL-2-stimulated lymphocytes from the same individual. In two cases, the CTL lines were cytotoxic for the IL-2-stimulated lymphoblasts as well as the leukemic cells. The generation of cytotoxic T cells appeared to be reproducible. These CTL lines may be used in a phase I/II trial for the treatment of relapsed leukemia after allogeneic BMT.