The correlation between c-kit downregulation on mobilized CD34⁺ cells and CD34⁺ cell yield (×10⁶/kg BW) of a 10-L apheresis performed on the same day in the authors' institution. Forty-two specimens from six mobilization protocols were analyzed. (□), Patients who received cyclophosphamide 4 to 7 g/m² intravenously and had blood cells harvested during recovery from myelosuppression. (✙), Patients who received IL-3 and GM-CSF at 5 μg/kg/d subcutaneously and had blood cells harvested during GM-CSF administration. (⊞), Chemotherapy naive patients who received G-CSF at 12 μg/kg subcutaneously daily and had blood cells harvested on days 5, 6, and 7. (○, ▵), Patients who received myelosuppressive chemotherapy and G-CSF or GM-CSF and had blood cells harvested during recovery from myelosuppression. (⊕), Chemotherapy naive patient who received G-CSF at 12 μg/kg subcutaneously daily and SCF at 5 to 15 μg/kg subcutaneously daily and had blood cells harvested on days 5, 6, and 7 of G-CSF administration. The correlation between c-kit downregulation and CD34⁺ cell yield per apheresis is significant at P << .005.