These data clearly indicate that suitable systems are presently available to isolate the human hematopoietic stem cell. The CFU-B1 and cells responsible for initiating long term hematopoiesis in vitro are likely identical since they share the same phenotype, CD34+DR-CD33-CD15-C71-. The suspension culture system as indicated above has several advantages over an adherent cell based long term marrow culture system. Use of this suspension culture system with 48 hourly feeding with recombinant cytokines will facilitate definition of the cytokine requirements for the human CFU-S.