Abstract: Objective: To investigate the immunogenity and immune regular ability of mesenchymal stem cells (MSCs) derived from rat bone marrow in mixed lymphocyte culture.Methods: MSCs and mixed lymphocyte reaction (MLR) cultures were set up.Splenic lymphocytes from SD rats were used as stimulator cells,and splenic lymphocytes from Wistar rat as responder cells.The study was composed of 6 groups.Group Ⅰ,the control group,was co-cultured with 1×10⁵ stimulator cells and 1×10⁵ responder cells,group Ⅱwith 1×10⁴ MSCs from SD rats with the same amount of stimulator cells as group Ⅰ,group Ⅲ with 1×10⁴ MSCs from SD rats and the same amount of stimulator and responder cells as group Ⅰ,group Ⅳ with 1×10⁴ MSCs from SD rats and the same amount of stimulator and responder cells as group Ⅰ plus 1-methyltryptophan (1-MT),group V with 1×10⁴ EPC from SD rats and the same amount of stimulator and responder cells as group Ⅰ plus phytohemagglutinin (PHA) as a mitogen,and group VI with 1×10³ MSCs and the same amount of stimulator and responder cells as group Ⅰ.The culture time was 120 hours for each group.The number of lymphocyte in each group was calculated with a liquid scintillation counter.The MSCs and the content of tryptophane were determined by TRP-HPLC.Results: MSCs inhibited T-lymphocytes proliferation of MLR culture in a dose-dependent manner.The content of tryptophan decreased greatly in MSCs and MLR cultures.1-MT reversed the effect of MSCs.Conclusions: MSCs could suppress T lymphocyte responses via IDO enzyme activity.