Objective:To explore whether the graft-versus-leukemia (GVL) effects could be enhanced and acute graft-versus-host disease (aGVHD) could be relieved by syngeneic bone marrow mixed with G-CSF-primed H-2 haploidentical marrow grafting.Methods:Female L615 (H-2k) mice were recipient mice and male (615×BALB/c) F1 (6BF1) (H-2k×H-2d) mice were donors respectively.Donor mice were injected subcutaneously with G-CSF daily at 0.01 μg/g for 6 days,and splenocytes were harvested on day 7.A total of 615 mice were loaded with L615 tumor cells and received 8.5 Gy (60Co γ-ray) irradiation three days later,followed by a mixed bone marrow transplantation (MBMT).The allo-grafts consisted of a mixture of syngenetic plus G-CSF-mobilized (control diluents) H-2 haploidetical marrow cells.GVL effects were monitored by survival time and survival rate of recipient mice.GVHD was assessed by clinical signs of weight loss,ruffled fur,diarrhea and histological changes of skin,liver and small intestines.Allogeneic chimerism was detected using cytogenetic analysis.Enzyme-linked immunosorbent assay (ELISA) method was used to detect cytokines (IL-2,IL-4 and IFN-γ).Fluorescence-activated cell sorting (FACS) analysis was used to detect T-cell phenotype.Results:(1) The mice merely received L615 leukemia cells were all died of leukemia.The L615 mice of 3∶1 and 4∶1 MBMT groups survived longer than those post syngeneic BMT (P＜0.01).(2) The survival time of mice in the G-CSF-treated MBMT groups was longer than that of non-primed MBMT groups (P＜0.05).Administration of G-CSF-treated 6BF1 mice could markedly increase the survival rate of 3:1 and 4:1 MBMT mice (P＜0.01) with little or a little GVHD.(3) As the post-transplanted time prolonged,the rates of allogeneic chimerism were decreased.The chimerism rates decreased to zero when the mice died of leukemia relapse.(4) L3T4+ cells and relative ratio in both subsets were significantly reduced in G-CSF-treated donor mice.After G-CSF-treated donors,splenocytes from recipients at day 14 post-MBMT showed an increased production of IL-4 and a decreased production of IL-2 and IFN-γ.Conclusion:Syngeneic bone marrow mixed with H-2 haploidentical marrow grafts had a potential way to increase GVL effects,and this GVL effects could be enhanced with little or a little GVHD by G-CSF preetreatment of donors.Improved survival in recipients of G-CSF-mobilized donors is associated with increased IL-4 production and decreased IL-2 and IFN-γ production.