The regenerative potential of bone marrow following exposure to relatively high doses of ionizing radiation, as well as the efficacy of hemopoietic growth factor treatment, are dependent on the residual number of hemopoietic stem cells. From studies in mice in particular, evidence has been obtained that immature hemopoietic stem cells are heterogenous with respect to repopulating capacity, with one subset being capable of short-term, transient hemopoietic reconstitution and another subset of sustained reconstitution. In rhesus monkeys, CD34+, RhLA-DRdull cells were identified as the small fraction of a bone marrow cell that contains reconstituting hemopoietic stem cells. The growth factor receptor phenotype of this immature cell fraction has been determined for granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin 3 (IL-3), and IL-6 as well as for kit-ligand, making c-kit an especially strong growth factor receptor marker for reconstituting stem cells. In addition, it is demonstrated that CD34+ cells appear in peripheral blood after exposure to radiation and are correlated to numbers of CD34+ cells in bone marrow. This finding suggests that circulating CD34+ cells may be used as a cellular marker with prognostic significance for both the number of residual stem cells as well as regeneration of immature hemopoietic cells in bone marrow.