objective To investigate the effect of bone marrowmesenchymal stem cells (MSC) on hematopoietic recoveryand acute graft-versus-host disease (GVHD) in a murineallogeneic umbilical cord blood transplantation (allo-UCBT) model.method MSCs were obtained from C57/BL mousebone marrow.The MSC phenotypes were identified byflow cytometry (FCM), and their ability to differentiate intoosteoblasts and adipocytes was tested.Once murine allo-UCBT and aGVHD models were established, mice weredivided into five groups: (1) total body irradiation (TBI)group, each mouse receiving 0.3 ml sterile saline infusionafter TBI and used as control;(2) UCB group,receiving2 9 106 umbilical cord blood mononuclear cells (UCBMNC)after TBI;(3) UCB?MSC group, receiving 2 9 106UCB-MNC and 2 9 107 MSC after TBI;(4) UCB?SCgroup, receiving 2 9 106 UCB-MNC and 2 9 106 spleencells after TBI;and (5) UCB?SC?MSC group, receiving2 9 106 UCB-MNC, 2 9 107 MSC and 2 9 106 spleencells after TBI.To evaluate the engraftment of HSC, thewhite blood cells, red blood cells, and platelets counts weretested at different time points after transplantation, and theratio of chimerism was identified by FCM.