Human SCID repopulating cells (SRC) are defined based on their functional ability to repopulate the bone marrow of NOD/SCID mice with both myeloid and lymphoid cell populations. The frequency of SRC in umbilical cord blood cells is 1 in 9.3 × 10⁵mononuclear cells. We report that as few as 8 × 10⁴ human cord blood mononuclear cells transplanted into NOD/SCID/B2m^null mice resulted in mutlilineage differentiation in the murine bone marrow, revealing a more than 11-fold higher SRC frequency than in NOD/SCID mice. Moreover, as few as 2 to 5 × 10³ CD34⁺ cells recovered from the bone marrow of primary transplanted NOD/SCID mice were sufficient for engrafting secondary NOD/SCID/B2m^null mice with SRC, suggesting SRC self-renewal. Thus, by using NOD/SCID/B2m^null mice as recipients, we established a functional assay for human stem cells capable of engrafting the bone marrow of primary and secondary transplanted immune-deficient mice with SRC, providing a model that better resembles autologous stem cell transplantation.