Low reactive oxygen species (ROS) levels are well-established characteristics of mouse hematopoietic stem cells (HSCs). However, little is known about these levels in human HSCs. This study aimed at quantifying ROS levels in human CD34 CD38 and CD34 CD38 human progenitors from bone marrow, cord blood and cells mobilized for autologous HSC transplantation. A specifically devised multiparameter flow cytometry method was used to quantify ROS levels (H DCFDA staining) in sub-populations of primary cells. Results were confirmed by assessing gene expression level of the ROS scavenger GPX3, a key determinant of HSC self-renewal, in sorted CD34 CD38 and CD34 CD38 cells. CD34 CD38 cells from bone marrow and cord blood displayed significantly lower levels of ROS than CD34 CD38 cells and other leukocytes. Moreover, the correlation between ROS and GPX3 expression was verified in sorted CD34 CD38 and CD34 CD38 cells. These results confirm, in human, data previously reported in mice. Moreover, the flow cytometry assay we developed could allow for a more precise enumeration of repopulating primitive progenitors in the course of HSC transplantation.