This study investigated the correlation between and compared the effects of reactive oxygen species (ROS) and p38 mitogen-activated protein kinase a (p38MAPKα) in the ex vivo expanded umbilical cord blood (hUCB) CD133+ cells.hUCB CD133+ cells were cultured in the hematopoietic stem cells (HSCs) culture medium with N-acetylcysteine (NAC,an anti-oxidant),p38MAPKα-specific inhibitor (SB203580) or their combination.The levels of ROS and expression of phosphorylated p38MAPKα (p-p38) in CD133+ cells were flow cytometrically detected.The efficacy of ex vivo expansion was evaluated by the density of CD 133+ cell sub-group colony-forming cells (CFC) and cobblestone area-forming cells (CAFC) assay.Our results showed decreased ROS levels in NAC,SB203580,and their combination treatment groups were almost 37％,48％,and 85％,respectively.Furthermore,SB203580 abrogated the activation of p38MAPKα more obviously than NAC.Moreover,the CD133+ cells in SB203580 treatment group had a 21.93±1.36-fold increase,and 14.50±1.19-fold increase in NAC treatment group,but only 10.13±0.57-fold increase in control group.In addition,SB203580 treatment led a higher level increase in the number of CFU and CAFC than NAC did.These findings suggested that,in expanded CD133+ cells,ROS activates p38MAPKα,which,in turn,induces ROS production,and p38MAPKα might be the most suitable regulator in ROS- p38MAPKα pathway for the promotion ofHSCs ex vivo expansion.