We and others have reported that human hematopoietic stem cells (HSCs) are also present in the CD34-negative (CD34(-)) fraction of human cord blood (CB). Here, we examined the hematopoietic engraftment potential of 13 or 18 lineage-negative (13Lin(-) or 18Lin(-)) CD34(+/-) cells from human CB in mice and sheep. Both 13Lin(-) and 18Lin(-) CD34(+) cells efficiently engrafted in mice irrespective of transplantation route, be it by tail-vein injection (TVI) or by intra-bone marrow injection (IBMI). These cells also engrafted in sheep after in utero fetal intra-hepatic injection (IHI). In contrast, neither 13Lin(-) nor 18Lin(-) CD34(-) cells engrafted in either mice or sheep when transplanted by regular routes (i.e., TVI and fetal IHI, respectively), although both 13Lin(-) and 18Lin(-) CD34(-) cells engrafted in mice when transplanted by IBMI and exhibited multilineage reconstitution ability. Thus, the homing ability of CD34(-) HSCs is significantly more limited than that of CD34(+) HSCs. As for 18Lin(-), CD34(-) HSCs are characterized by low expression of the tetraspanin CD9, which promotes homing, and high expression of the peptidase CD26, which inhibits homing. This unique expression pattern homing-related molecules on CD34(-) HSCs could thus explain in part their reduced ability to home to the BM niche.