The only conclusive method to assay stem cells is to follow their ability to repopulate conditioned recipients, making it difficult to study human stem cells. The development of systems to transplant human hematopoietic cells into immune-deficient mice lays the foundation for such an experimental repopulation assay for primitive human cells. Cell purification and gene marking studies have shown that the repopulating cells, termed severe-combined immunodeficiency (SCID) mouse-repopulating cells (SRC), are primitive and distinct from most of the progenitors that are detected using short and long-term in vitro culture assays. The SRC are exclusively CD34+CD38- and poorly infected with retrovirus vectors. These gene marking data are reminiscent of the human clinical trials establishing that the SRC assay is a good surrogate to develop improved transduction methods. Limiting dilution analysis has been used to establish a quantitative assay for SRC that can be used to precisely determine the effect of various cytokine cocktails on the proliferation and differentiation of SRC during in vitro culture.