We have studied the expression, localization, and function of the ABCG2 transporter, a universal stem cell marker, at the protein level in human cultured neural stem/progenitor cells (hNSPCs) using immunoblotting, immunofluorescence, and ATPase assays. Human NSPCs were isolated from human fetal brain and propagated in vitro as neurospheres. Both the cells in neurospheres and single cells dissociated from neurospheres showed high levels of ABCG2, and about 63% of the cells in neurospheres were ABCG2-positive, similar to the proportion of nestin-positive cells, and in most cases the ABCG2 and nestin staining co-localized in the same cells. Both the three-dimensional structure of single hNSPCs stained with anti-ABCG2 antibodies and an examination using a biochemical marker for the plasma membrane indicated that ABCG2 was localized to the plasma membrane of hNSPCs. The ABCG2 expressed in hNSPCs had prazosin-sensitive ATP hydrolysis activity, and the ABCG2 level was sharply down-regulated during hNSPC differentiation. All these results suggested that ABCG2, was functionally expressed in hNSPCs. ABCG2 might play a significant role in maintaining human neural stem cells in an undifferentiated state and in protecting hNSPCs from xenobiotics or other toxic substances in vivo.