Abstract: Objective: Mesenchymal stem cells (MSCs) are low-immunogenic cells with immunomodulatory capacity, which show promise in the treatment of immune system related diseases. However, whether MSCs maintain immunomodu-latory function in a pathological environment is not clear. The aim of this study was to investigate the immunomodulatory function of MSCs derived from bone marrow of CML patients and to compare it with that of MSCs from normal donors. Methods:MSCs were isolated from CML and normal donors, then the T lymphocyte activation, proliferation and suppres-sion were compared. Results: Compared with those of MSCs from normal bone marrow, the capacity of suppressing T cell proliferation and activation of MSCs derived from bone marrow of CML patients were weakened. The inhibitory effect on apoptosis was increased. MSCs derived from bone marrow of CML patients persistently displayed fibroblast-like morpholo-gy and isotype analysis indicated that they were all persistently negative for CD 34and CD31but positive for Flk1, CD29, CD44and CD105 . They could inhibit T lymphocyte activation and the percentages of CD 25, CD69and CD44in PHA in -duced T lymphocytes were ( 12.3 ± 3.5)% , ( 34.5 ± 5.9)% and (29.4 ± 7.0)% , respectively, but when co-cultured with normal MSCs were ( 3.1 ± 2.3)%, (6.4 ± 3.2)% and ( 2.1 ± 1.7)% and when co-cultured with CML MSCs were ( 5.4 ± 2.3)%, (31.5 ± 6.8)% and (24.5 ± 3.6)% , respectively. Conclusion:CML hemangioblasts can inhibit T lymphocyte proliferation and activation and promote T lymphocyte apoptosis, indicating immuno-defects of CML hemangioblasts. MSCs from the pathological environ-ment should not be used for autologous transplantation.