A major goal of current hematopoiesis research is to develop in vitro methods suitable for the measurement and characterization of stem cells with long-term in vivo repopulating potential. Previous studies from several centers have suggested the presence in normal human or murine marrow of a population of very primitive cells that are biologically, physically, and pharmacologically different from cells detectable by short-term colony assays and that can give rise to the latter in long-term cultures (LTCs) containing a competent stromal cell layer. In this report, we show that such cultures can be used to provide a quantitative assay for human "LTC-initiating cells" based on an assessment of the number of clonogenic cells present after 5-8 weeks. Production of derivative clonogenic cells is shown to be absolutely dependent on the presence of a stromal cell feeder. When this requirement is met, the clonogenic cell output (determined by assessment of 5-week-old cultures) is linearly related to the input cell number over a wide range of cell concentrations. Using limiting dilution analysis techniques, we have established the frequency of LTC-initiating cells in normal human marrow to be approximately 1 per 2 X 10(4) cells and in a highly purified CD34-positive subpopulation to be approximately 1 per 50-100 cells. The proliferative capacity exhibited by individual LTC-initiating cells cultured under apparently identical culture conditions was found to be highly variable. Values for the number of clonogenic cells per LTC-initiating cell in 5-week-old cultures ranged from 1 to 30 (the average being 4) with similar levels being detected in positive 8-week-old cultures. Some LTC-initiating cells are multipotent as evidenced by their generation of erythroid as well as granulopoietic progeny. The availability of a system for quantitative analysis of the proliferative and differentiative behavior of this newly defined compartment of primitive human hematopoietic cells should facilitate future studies of specific genetic or microenvironmental parameters involved in the regulation of these cells.