Here we describe the in vitro generation of a novel adherent cell fraction derived from highly enriched, mobilized CD133(+) peripheral blood cells after their culture with Flt3/Flk2 ligand and interleukin-6 for 3 to 5 weeks. These cells lack markers of hematopoietic stem cells, endothelial cells, mesenchymal cells, dendritic cells, and stromal fibroblasts. However, all adherent cells expressed the adhesion molecules VE-cadherin, CD54, and CD44. They were also positive for CD164 and CD172a (signal regulatory protein-alpha) and for a stem cell antigen defined by the recently described antibody W7C5. Adherent cells can either spontaneously or upon stimulation with stem cell factor give rise to a transplantable, nonadherent CD133(+)CD34(-) stem cell subset. These cells do not generate in vitro hematopoietic colonies. However, their transplantation into nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice induced substantially higher long-term multilineage engraftment compared with that of freshly isolated CD34(+) cells, suggesting that these cells are highly enriched in SCID-repopulating cells. In addition to cells of the myeloid lineage, nonadherent CD34(-) cells were able to give rise to human cells with B-, T-, and natural killer-cell phenotype. Hence, these cells possess a distinct in vivo differentiation potential compared with that of CD34(+) stem cells and may therefore provide an alternative to CD34(+) progenitor cells for transplantation.