The optimal time for the harvesting of peripheral blood stem cells following chemotherapy and growth factors for autologous transplantation is based on the CD34 cell count. In this study, 51 patients having 59 stem cell mobilizations were assessed for the timing of the harvest by a CD34 cell count and an immature reticulocyte fraction (IRF). Results from 272 preharvest tests showed that when the CD34 cells were not harvestable, defined as a CD34 cell count of <15 cells/μl, the IRF was always ≤0.2. A low IRF resulted in a negative predictive value of 1 for the harvesting of stem cells. The IRF is therefore a valuable negative predictor of the timing of autologous stem cell harvesting.