Clinical and experimental data suggest a role for the immune response in preventing leukemic relapses after allogeneic bone marrow transplantation (BMT): the graft-versus-leukemia (GVL) effect. In this report, we have evaluated the response of normal donor lymphocytes against allogeneic leukemic cells as an in vitro model of the GVL effect. We used a limiting dilution technique in order to determine the frequency of cytotoxic T-lymphocyte precursors (pre-CTL) against allogeneic leukemic blasts among normal donor lymphocytes. We demonstrate a considerable variability of CTL precursor frequency. This variability depended on leukemic populations since, for a given leukemia, the pre-CTL frequency was comparable among our tested normal allogeneic donors. Moreover, when HLA-DR negative leukemias were used as allostimulators, the pre-CTL frequencies were extremely low. In order to verify the impact of leukemic DR expression on the stimulatory capacity of leukemic cells, we selected and analyzed in mixed lymphocyte tumor cell culture (MLTC), a panel of myelogenous and lymphoblastic leukemias with variable levels of DR expression, each against different allogeneic responders. Our results demonstrated a close correlation (r = 0.953, p < 0.0001) between the proliferative response of alloactivated lymphocytes and the percentage of stimulatory leukemic cells expressing HLA-DR molecules. Anti-MHC class II monoclonal antibodies inhibited the lymphocyte proliferation in the MLTC, confirming the preponderant role of DR in the generation of this response. Overall, our results demonstrate the extreme variability of leukemic cells in their allostimulatory capacity and the central role of DR expression in determining leukemic allo-recognition. In the setting of a clinical protocol, our data suggest that the infusion of allogeneic T lymphocytes in a DR negative leukemia will not lead to an alloreactive T-cell anti-tumor effect.