Patients transplanted with mobilized blood progenitor cells (PBPC) recover their neutrophil counts more rapidly than patients transplanted with bone marrow even when they receive the same dose/kg of granulocyte-macrophage colony-forming cells (CFU-GM). Here we have sought a biological explanation for this phenomenon. Most CD34-positive PBPC are quiescent (<1% in S phase) when they are collected from the bloodstream of patients treated with cyclophosphamide and granulocyte colony-stimulating factor (G-CSF), but we have shown that they are able to resume proliferation rapidly in vitro by measuring the kinetics of CFU-GM production by primitive plastic-adherent (PΔ) cells. Also, PΔcells in PBPC harvests, unlike normal marrow PΔ cells, were insensitive to cell-cycle restraint imposed by contact with marrow-derived stromal cells. We found that PΔ cells in PBPC collections produce relatively more CFU-GM and relatively fewer BFU-E than PΔ cells in bone marrow, indicating that granulopoiesis might occur at the expense of erythropoiesis, but we were unable to find any differences in the kinetics of granulocytic maturation between PBPC and bone marrow. Our interpretation of these findings is that transplanted PBPC rapidly enter the cell cycle and contact with stromal cells in the marrow does not reduce the proportion of progenitors participating in neutrophil production. Consequently, neutrophil recovery after PBPC infusion is more rapid than neutrophil recovery after marrow infusion. Granulopoiesis at the expense of erythropoiesis may also contribute to this effect.