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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 CD8alpha(+)CD3epsilon(+) 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 stem-cell transplantation.