Effects of intermittent-flow centrifugation leukapheresis (IFCL) on leukocyte counts in donor blood have not been reported. Accordingly, total and differential white blood cell counts were preformed serially during IFCL. A significant decrease (p less than 0.05) in all types of leukocytes occurred in donor blood early in pheresis and was probably the consequence of hemodilution. Concentrations of most leukocyte types remained decreased throughout the procedure, and a rebound leukocytosis was not detected. Thus, leukocyte kinetics in donor blood during IFCL differ from those reported for filtration leukapheresis in which the pattern of selective neutropenia followed by rebound neutrophilia has been ascribed to complement activation.