Hematopoietic stem cells (HSCs) undergo self-renewal to maintain hematopoietic homeostasis for lifetime, which is regulated by the bone marrow (BM) microenvironment. The chemokine receptor CXCR4 and its ligand CXCL12 are critical factors supporting quiescence and BM retention of HSCs. Here, we report an unknown function of CXCR4/CXCL12 axis in the protection of HSCs against oxidative stress. Disruption of CXCR4 receptor in mice leads to increased endogenous production of reactive oxygen species (ROS), resulting in p38 MAPK activation, increased DNA double-strand breaks and apoptosis leading to marked reduction in HSC repopulating potential. Increased ROS levels are directly responsible for exhaustion of the HSC pool and are not linked to loss of quiescence of CXCR4-deficient HSCs. Furthermore, we report that CXCL12 has a direct rescue effect on oxidative stress-induced HSC damage at the mitochondrial level. These data highlight the importance of CXCR4/CXCL12 axis in the regulation of lifespan of HSCs by limiting ROS generation and genotoxic stress. Reactive oxygen species (ROS) are produced during oxidative respiration or through exogenous environmental stresses, such as ionizing radiations or genotoxic treatments. Physiological concentrations of ROS play a role in signal transduction 1,2 , but at high concentrations they can oxidize cell constituents leading to protein carbonyla-tion, lipid peroxidation and DNA damage that activate multiple apoptosis pathways 3. By far, the most important source for ROS is mitochondria 4,5 and their endogenous production as by-products of aerobic respiration is thought to be the cause of most oxidative damages observed in mammals and particularly during aging 6,7. To avoid accumulation of oxidative stress, cells have evolved mechanisms to fine-tune ROS levels. They involve distinct groups of specialized proteins such as superoxide dismutase (SOD), catalase and glutathione peroxidase. Reduced glutathione (GSH), which also exists in the cell in its oxidized form (GSSG), is considered as the most abundant molecule among endogenous antioxidants. Alteration in its redox status serves as an indicator of oxida-tive stress when antioxidant defense mechanisms are not completely efficient and is a common feature of ageing and many pathological situations including AIDS, neurodegenerative diseases and cancer. Hematopoietic stem cells (HSCs) are defined as cells capable of both self-renewal and differentiation into any of the hematopoietic cell lineages, properties that allow hematopoietic reconstitution 8,9. Long-term maintenance of HSCs is precisely regulated by the equilibrium between proliferation and quiescence to maintain their numbers