CD34 is the first documented cell surface antigen whose expression within the hematopoetic system is restricted to stem and progenitor cells of all lineages. Transplantation of CD34(+) cells has created a need for accurate and rapid assessment of CD34(+) cell concentrations in peripheral blood as well as for the optimal timing of apheresis sessions to insure the harvest of sufficient CD34(+) cells. However, accurate enumeration of these cells represents a rare-event analysis, a demanding and somewhat complicated procedure. This comprehensive unit presents the clearest and most definitive methods yet available for CD34(+) enumeration by flow cytometry, evaluates a number of commercially available procedures, and provides invaluable supporting information and detailed analysis which we believe is found in no other publication. All the possible problems are discussed and examples provided, allowing the clinical hematology lab to be certain that analysis of CD34(+) cells is performed in an appropriate manner.