The nonobese diabetic/severe combined immune deficiency (NOD-scid) xenotransplantation model is the "gold standard" for assaying human hematopoietic stem cell activity. Systematic advancements, such as depletion of natural killer cell activity with anti-CD122 antibody, direct intrafemoral injection, and deletion or truncation of IL2Rgamma, have improved human cell engraftment; however, questions remain whether these mouse models are equivalent or, if not, which model is superior for assaying hematopoietic stem cell activity. To address this, we compared overall engraftment and multilineage differentiation of near-limiting doses of lineage-depleted human umbilical cord blood cells by direct intrafemoral injection into NOD/Lt-scid, NOD/Shi-scid, NOD/Lt-scid/IL2Rgamma(null) (NSG), and NOD/Shi-scid/IL2Rgamma(null) mice. Transplantation into NSG mice generated moderately higher human engraftment levels in bone marrow compared with other strains. At limiting doses, NSG mice of both sexes were 3.6-fold more sensitive in detecting SCID-repopulating cells compared with NOD/Lt-scid mice. However, NSG females exhibited higher engraftment at limiting cell doses, resulting in an overall increase in SCID-repopulating cell detection of 9-fold. Both NSG and NOD/Shi-scid/IL2Rgamma(null) support significantly improved engraftment in peripheral tissues compared with NOD/Lt-scid and NOD/Shi-scid mice, whereas NSG mice provide greater human engraftment in bone marrow than all other strains, especially at limiting doses.