Clinical observations in patients undergoing bone marrow transplantation implicate the involvement of CD8+ cells in promoting the stem-cell engraftment process. These findings are supported by mouse transplant studies, which attributed the engraftment-facilitating function to subpopulations of murine CD8+ cells, but the analogous cells in humans have not been identified. Here, we report that clinical stem-cell grafts contain a population of CD8α+CD3ε+ T-cell receptor-negative cells with an engraftment facilitating function, named candidate facilitating cells (cFCs). Purified cFC augmented human hematopoiesis in NOD/SCID mice receiving suboptimal doses of human CD34+ cells. In vitro, cFCs cocultured with CD34+ cells increased hematopoietic colony formation, suggesting a direct effect on clonogenic precursors. These results provide evidence for the existence of rare human CD8+CD3+TCR- cells with engraftment facilitating properties, the adoptive transfer of which could improve the therapeutic outcome of stemcell transplantation.