A major problem hampering effective stem cell–based therapies is the absence of a clear understanding of the human hematopoietic stem cell (HSC) pool composition. The severe combined immunodeficiency (SCID) repopulating cell (SRC) xenotransplant assay system provides a powerful tool for characterizing the frequency, cell surface markers, cell cycle status, homing and response to cytokine stimulation of human HSCs^,,. Clonal tracking of retrovirally transduced SRCs and transplantation of specific subpopulations revealed SRC classes with distinct repopulation potentials^,,,. However, all HSC repopulation assays are based on intravenous injection, a complex process that requires circulation through blood, recognition and extravasation through bone marrow vasculature, and migration to a supportive microenvironment^,,,. Thus, some classes of HSCs may remain undetected. By direct intrafemoral injection, we identified rapid SRCs (R-SRCs) within the Lin⁻CD34⁺CD38^loCD36⁻ subpopulation. R-SRCs rapidly generate high levels of human myeloid and erythroid cells within the injected femur, migrate to the blood and colonize individual bones of non-obese diabetic (NOD)-SCID mice within 2 weeks after transplantation. Lentivector-mediated clonal analysis of individual R-SRCs revealed heterogeneity in their proliferative and migratory properties. The identification of a new HSC class and an effective intrafemoral assay provide the tools required to develop more effective stem cell–based therapies that rely on rapid reconstitution.