Peripheral blood stem cells are increasingly used to ensure rapid haematological engraftment after myeloablative chemotherapy. After mobilization, progenitor cells in the blood can be enumerated to predict an adequate collection by leukapheresis. The Advia 120 automated counter has an immature cell channel measuring a parameter known as large undifferentiated cells (LUC's), which were quantified to assess their value in refining the timing of apheresis. Data were available from 102 apheresis sessions. Positive correlation was found for peripheral blood CD34+ cells and apheresis counts (r = 0.82, P < 0.0005) but not for total WCC (r = -0.15, P = 0.13) or LUC count (r = 0.12, P = 0.23). Our results indicate that the LUC population in peripheral blood has no relevance to the subsequent CD34 content of the apheresis product and CD34 cell enumeration by flow cytometry is advocated.