Abstract: AC133 (CD133) is a highly conserved antigen expressed on hematopoietic stem cells with unknown function. In order to further characterize CD133+ progenitor cells, we purified CD133+ stem cells using the method of magnetic activated cell sorting (MACS) from healthy adult volunteers mobilized with granulocyte colony-stimulating growth factor (G-CSF) to a mean purity of 94%. The purified CD133+ cells highly engrafted NOD/SCID mice. In addition, unseparated mononuclear cells or CD133+ stem cells isolated from the bone marrow of transplanted NOD/SCID mice gave rise to engraftment of secondary recipients. Upon ex vivo culture of purified CD133+ cells with FLT3/Flk2 ligand (FL) and interleukin-6 (IL-6), a plastic-adherent cell population could be observed after 6 weeks in culture. These adherent cells did not express CD34 or CD133 antigens on their surface, nor did they express markers for endothelial, mesenchymal, or dendritic cells. After incubation of these adherent cells with stem cell factor (SCF), non-adherent cells were observed which partially co-expressed CD133, but were negative for CD34. These nonadherent CD34− cells showed a high engraftment capacity in NOD/SCID mice. From our results, we conclude that CD133 might be a marker of early progenitors with a high NOD/SCID engraftment potential. The fact that CD133+ hematopoietic progenitors can give rise to an adherent population which is CD133− and CD34− and that these cells can again give rise to a CD133+CD34− stem cell population with high NOD/SCID engraftment potential indicates the plasticity of hematopoietic precursors. CD133+ stem cells might be useful for research and for clinical application.