Filtration leukapheresis (FL) in which donors are pretreated with steroids, induces a rapid neutropenia followed by neutrophilia. To investigate whether these phenomena are associated with changes in circulating progenitor cells (CFU-GM and BFU-E), 5 donors who underwent FL following steroid administration were compared with a control group who received steroids alone. Steroids alone caused an initial reduction in both circulating lymphocytes and committed progenitor cells followed by a rebound above baseline. Among donors undergoing FL, the changes in lymphocyte counts were identical to the steroid controls and the same early suppression of progenitor cells also occurred, but the rebound was blunted. Thus, the neutrophilia that occurs as a result of the FL procedure is not associated with an increase in circulating progenitor cells; and hence, the procedure does not appear to be a useful adjunct for increasing the yield of hematopoietic stem cells from the peripheral blood.