Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is one of the most valid methods for the treatment of malignant and refractory haematological diseases. Graft versus host disease (GVHD), which remains the main complication after allo-HSCT, leads to the high transplant-related mortality. Therefore, how to lower the incidence rate of GVHD is the hot topic in the field of modern transplantation immunity.The regulatory T cells (CD4~ CD25~ T_(Reg)) are a group of T cells with idio-phenotype and special function, which are origined in thymus gland and can induce immunologic tolerance in vivo. TGF-|3 secreted by T_(Reg) cells can combines with their own plasmalemma receptor, and then educe depressive effect by cell-cell contact after activated. The activated T_(Reg) cells can combine together with TGF-P specific antibody of the high density, and obstruct immunosuppression effect of T_(Reg) cells. Moreover T_(Reg) cells can restrain the generation of CD4~ CD25~- and CD8~  T cells activated by CD3 specific antigen and inhibit the secretion of related cytokines. T_(Reg) cells can also educe depressive effect by cell indirect contact. Competitive inhibition is another path by which T_(Reg) cells can act as suppressive cells. So T_(Reg) cells are a independent cell population which contributes to keeping organism's immunological homeostasis.Objectives:In this research, we explored whether GVHD of murine after allo-BMT are