A routine clinical assay for hematopoietic stem/progenitor cells must meet the following criteria.1. Simplicity—to allow widespread applicability2. High sensitivity—since hematopoietic stem cells are rare events3. Accuracy—to provide clinically relevant results4. Reproducibility—to provide clinically reliable results5. Speed—to provide real-time and on-line analysisTo meet these criteria, we selected a flow cytometric assay using whole blood or apheresis products, commercially available, directly conjugated fluorochrome- labeled monoclonal antibodies, routine staining/whole blood lysing procedures, and a simple gating strategy using light scatter and two-color immunofluorescence analysis using a pan-CD45 FITC/pan-CD34 PE monoclonal antibody combination (25). This procedure meets the requirements as a simple assay that can be performed in clinical flow cytometry laboratories.