Previous studies have demonstrated that T cell-depleted (TCD) syngeneic marrow protects against graft-versus-host disease (GVHD) when given along with an allogeneic lymphocyte plus bone marrow (BM) inoculum to lethally irradiated mice. In spite of this anti-GVHD effect, TCD syngeneic marrow is ultimately eliminated by non-TCD allogeneic marrow, permitting complete allogeneic reconstitution. These observations suggested that allogeneic BM might also eliminate host-type leukemic cells in a model in which TCD syngeneic marrow is co-administered to provide protection from GVHD. In the present studies, we describe the establishment of a new model using the EL4 leukemia/lymphoma. Lethally irradiated B10 (H-2b) mice were given a lethal dose of EL4 cells (H-2b) along with syngeneic marrow or a mixture of TCD syngeneic plus non-TCD allogeneic (B10. D2, H-2d) marrow. Non-TCD allogeneic marrow, in contrast to TCD or unmanipulated syngeneic marrow, delayed or prevented mortality from the otherwise lethal EL4 inoculum, without producing clinically apparent GVHD. The anti-leukemic effect of allogeneic marrow alone was not attenuated by the co-administration of TCD syngeneic marrow, and such animals repopulated as completely allogeneic chimeras. Similar anti-leukemic effects of mixed marrow inocula in a haploidentical strain combination, and an anti-leukemic effect against established tumor were also demonstrated. This model may have the potential to increase the safety of clinical bone marrow transplantation across greater HLA disparities, while permitting utilization of the anti-leukemic and alloengraftment-promoting effects of T cells in allogeneic marrow inocula.