Objective: A murine model of mixing syngeneic and haploidentical major histocompatibility complex (MHC) matched bone marrow cells transplant was used to evaluate the effect of splenocytes in graft-versus-host disease (GVHD) and host-versus-graft reaction (HVGR). Methods: BALB/C recipient mice were lethally conditioned with 8.5 Gy and injected with different grafts which consisted of syngeneic bone marrow cells plus splenocytes (SPLCs) and haploidentical MHC matched bone marrow cells (BMCs)plus different doses of splenocytes. Recipient mice were detected for the percentage of haploidentical MHC matched mouse origin cells in the peripheral blood cells and checked daily for the appearance of GVHD symptoms. Histopathological examination of multiple organs from moribund mice was used to evaluate the grades of GVHD. Results: Recipient mice infused with 10 × 106 haploidentical MHC matched SPLCs and 5×106 syngeneic splenocytes showed a higher level and more stable chimerism with GVHD Ⅱ degree histopathological alterations. Histopathological results of GVHD in other groups hosts were not obvious, and the levels of chimerism were unstable. All of the mice survived over 150 d. Conclusion:The proportion and dose of syngeneic and haploidentical MHC splenocytes are of importance for inducing stable engraftment on the basis of nonlethal GVHD and to balance GVHD and HVGR.