Activation of complement cascade (ComC) play and important role in mobilization of hematopoietic stem/progenitor cells (HSPCs) from bone marrow (BM) into peripheral blood (PB). While there are vast experimental data on the mechanisms and factors that induce or promote mobilization of HSPCs, there is relatively less data on negative regulators of this process. We demonstrate for the first time that heme oxygenase-1 (HO-1) that has a well-documented anti-inflammatory potential plays an important and heretofore unrecognized role in retention of HSPCs in BM niches by i) modulating negatively activation of mobilization promoting ComC, ii) maintaining stromal derived factor-1 (SDF-1) level in the BM microenvironment and iii) attenuating chemotactic responsiveness of HSPCs to SDF-1 and sphingosine-1 phosphate (S1P) gradients in PB. Furthermore, our data showing a positive mobilizing effect by a non-toxic small-molecule inhibitor of HO-1 (SnPP) suggest that blockade of HO-1 would be a promising strategy to facilitate mobilization of HSPCs. Further studies are also needed to evaluate better the molecular mechanisms responsible for the potential effect of HO-1 in homing of HSPCs after transplantation.