AbstractThe collection efficiency (CE) of the Fenwall CS3000 continuous flow blood cell separator in the apheresis of peripheral blood stem cells during haemopoietic recovery following myelosuppressive chemotherapy was analysed. Ninety‐three aphereses were performed in 19 patients using procedure 3 on the Fenwal CS3000. The overall CE was calculated from the pre‐apheresis cell counts and the stated blood volume processed. Instantaneous CE was calculated from cell counts in the inlet and return lines. The overall mononuclear cell and granulocyte‐macrophage colony forming unit CE were 64.0% and 55.8%, respectively, significantly lower than the instantaneous CEs of 94.5% and 95.4%, respectively (P = 0.0001, t test, for both comparisons). Three factors unrelated to machine performance contributed to the lower overall CE despite a high instantaneous CE: (I) A fall in the patient's mononuclear cell counts during apheresis leading to an overestimation of the cells available for collection, (2) dilution of blood by anti‐coagulant, and (3) the operational dead space of the Fenwal CS3000. The overall CE corrected for these 3 factors approximated the instantaneous CE closely. Thus there is little room for further enhancement of machine performance because the Fenwal CS3000 is already operating with a very high instantaneous CE. To achieve major improvement in the yield of peripheral blood stem cell harvests, more effective mobilization protocols and better timing of apheresis are required. © 1992 Wiley