Mononuclear cell fractions of human blood and plateletpheresis residues were compared for their content of hemopoietic precursor cells. Erythroid burst‐forming units (BFU‐E) averaged 560 ± 130 per ml of blood and granulocyte‐monocyte colony forming units (CFU‐C) averaged 240 ± 90 per ml blood. Estimates based on a blood volume of 7 per cent of body weight indicate that the total blood pools of BFU‐E and CFU‐C are about 3.5 × 106and 1.5 × 106cells respectively. Plateletpheresis residues prepared after a mean of 3.4 liters of blood had been processed contained 1.3±0.22× 106BFU‐E and 0.50 ± 0.13 × 106CFU‐C. These values represent a 68 and 63 per cent recovery respectively of BFU‐E and CFU‐C from the processed 3.4 liters of blood. Sequential studies were performed over three days following one plateletpheresis in four donors. CFU‐C and BFU‐E approximately doubled between 48 and 72 hours after a plateletpheresis. During this time there was no significant alteration in the per cent of null, T‐ or B‐lymphocytes in blood. Thus, plateletpheresis appears to lead to a mobilization of precursor cells, which results in a transient increase in their concentration in blood. Therefore, cytapheresis 48 to 72 hours after an initial short‐term procedure could harvest much larger numbers of precursor cells. Moreover, such techniques would put blood precursor cell content of plateletpheresis residues within reach of the precursor cell content in the volume of