Objective(s):Immunosuppressive property of mesenchymal stem cells (MSCs) has great attraction in regenerative medicine especially when dealing with tissue damage involving immune reactions. The most attractive tissue sources of human MSCs used in clinical applications are bone marrow (BM), adipose tissue (AT), and Wharton's jelly (WJ) of human umbilical cord. The current study has compared immunomodulatory properties of human BM, AT, and WJ-MSCs. Materials and Methods: Three different types of human MSCs were isolated, cultured, and characterized by flow cytometry and differentiation potentials. The MSCs were co-cultured with allogeneic phytohemagglutinin (PHA) activated peripheral blood mononuclear cells (PBMCs). The proliferation of PBMCs was assessed by flow cytometry of carboxyfluorescein succinimidyl ester (CFSE) stained cells and compared to each other and to the growth of PBMCs in the absence of MSCs, 3 days post co-culture. Additionally, the growth suppression was indirectly assessed by using the transwell culture system. Results: the proliferation of PBMCs reduced to 6.2, 7 and 15.4- fold in cultures with AT-MSCs, WJ-MSCs, and BM-MSCs, respectively, compared to the PHA-activated cells. When the growth suppression was indirectly assessed by using the transwell culture system, it was revealed that AT-MSCs, WJ-MSCs, and BM-MSCs caused growth reduction in PBMCs to 3, 8, and 8 -fold, respectively, compared to the PHA-activated cells. Conclusion:These data collectively conclude that the immunomodulatory effects of MSCs, which may mostly carry out through direct cell to cell contact, are different between various sources. Accordingly results of this study may contribute to the application of these cells in cell therapy and regenerative medicine.