Peripheral blood progenitor cells (PBPC) reside within the mononuclear cell (MNC) component of the blood and can be collected using a number of apheresis devices, including the Fenwal CS3000 Plus Blood Cell Separator. Increased MNC collection efficiency, therefore, may reduce the number of apheresis required to achieve collection goals. In this study, patients were divided into groups by absolute MNC count to determine the effect of interface detector offset (I/O) adjustment on MNC collection efficiency. Apheresis products from 104 procedures collected using a standard I/O setting of 100 were compared with 121 collections for which the I/O setting was adjusted according to the preapheresis MNC count. Adjustment of the I/O setting in this manner had no statistically significant impact on the per kilogram dose of MNC collected. The data did show that MNC collection efficiency was reduced as both the MNC count and I/O setting increased, as the collection efficiency was greatest for patients with the lowest peripheral MNC counts and was inversely correlated with the preapheresis MNC count. Although contamination of the product with platelets was drastically reduced at higher I/O settings, there was a concomitant rise in RBC contamination. We conclude that a standard setting of 100 is preferable to adjustment of the I/O setting as a function of the preapheresis MNC count.