ABSTRACTHuman placenta-derived mononuclear cells (MNC) were isolated by a Percoll density gradient and cultured in mesen-chymal stem cell (MSC) maintenance medium. The homogenous layer of adherent cells exhibited a typical fibroblast-like morphology, a large expansive potential, and cell cycle characteristics including a subset of quiescent cells. In vitrodifferentiation assays showed the tripotential differentiation capacity of these cells toward adipogenic, osteogenic andchondrogenic lineages. Flow cytometry analyses and immunocytochemistry stain showed that placental MSC was ahomogeneous cell population devoid of hematopoietic cells, which uniformly expressed CD29, CD44, CD73, CD105,CD166, laminin, fibronectin and vimentin while being negative for expression of CD31, CD34, CD45 and α-smoothmuscle actin. Most importantly, immuno-phenotypic analyses demonstrated that these cells expressed class I majorhistocompatibility complex (MHC-I), but they did not express MHC-II molecules. Additionally these cells could sup-press umbilical cord blood (UCB) lymphocytes proliferation induced by cellular or nonspecific mitogenic stimuli. Thisstrongly implies that they may have potential application in allograft transplantation. Since placenta and UCB arehomogeneous, the MSC derived from human placenta can be transplanted combined with hematopoietic stem cells(HSC) from UCB to reduce the potential graft -versus-host disease (GVHD) in recipients. The homogenous layer of adherent cells exhibited a typical fibroblast-like morphology, a large expansive potential, and Cell cycle characteristics including a subset of quiescent cells. In vitrodifferentiation assays of the cells toward adipogenic, osteogenic and chondrogenic lineages. Flow cytometry analyzes and immunocytochemistry stain showed that placental MSC was ahomogeneous cell population devoid of hematopoietic cells, which are expressed Most importantly, immunohistochemical analysis of these cells expressed class I major histocompatibility complex (MHC), CD29, CD44, CD73, CD105, CD166, laminin, fibronectin and vimentin while being negative for expression of CD31, CD34, CD45 and α- -I), but they did not express MHC-II molecules. Addi tionally these cells could sup-press umbilical cord blood (UCB) lymphocytes proliferation induced by cellular or nonspecific mitogenic stimuli. Sincescently and they are potential application in allograft transplantation. Since placenta and UCB arehomogeneous, the MSC derived from human placenta can be transplanted combined with hematopoietic stem cells (HSC) from UCB to reduce the potential graft-versus-host disease (GVHD) in recipients.