The purpose was to study the effect of mesenchymal stem cell (MSC) on immune function, and to explore the new strategy to prevent graft versus host disease (GVHD) and host versus graft reaction (HVGR) in allogeneic bone marrow transplantation. MSCs from human bone marrow cells were isolated and cultured. The purity of MSCs were identified with the spindle-fibroblastic morphological characterization by microphotography, and the phenotypes were tested by flow cytometry. MSCs were planted in 6-well plates (8 x 10(4)/well for group A, 4 x 10(4)/well for group B and 2 x 10(4)/well for group C), and cocultured for 7 days with T cell isolated from peripheral blood. Peripheral blood T cells non-cocultured with MSC acted as the control group. CD3, CD4, CD8, and CD25 expressed on T cells were analyzed by flow cytometry after coculture with MSCs for 0, 24, 72 hours and 7 days respectively. The results showed that CD4(+)CD25(+) T cells and CD8(+) T cells of allogeneic T lymphocytes cocultured with bone marrow MSCs (group A and group B) increased obviously as compared with control group (T lymphocytes uncocultured with MSCs), and there were no difference between groups A and B. CD3, CD4, CD8 and CD25 expressed on T cells had no significant difference between experimental groups and control group. The result suggested that CD4(+)CD25(+)-regulatory T cells and CD8(+) T cells were increased apparently when cocultured with allogeneic MSCs. It is concluded that MSCs pretreatment may be useful in the prevention of GVHD and HVGR in allogeneic bone marrow transplantation and provides a new strategy to induce transplantation tolerance.