Effective transfusion therapy depends, in part, upon the ability of the transfused neutrophils to function normally in the recipient. To evaluate the in vivo function of neutrophils obtained with two widely used newer cell separators (Fenwal CS-3000 and IBM 2997), neutrophil concentrates from 12 normal subjects were labeled in vitro with 3H-diisopropylfluorophosphate, reinfused into the donor, and blood kinetics and skin chamber accumulation of the labeled cells were measured. Results were compared with previously obtained data (referred to as "normal") using neutrophils collected by phlebotomy. Blood kinetic measurements of cells obtained with the CS-3000 were normal, whereas blood recovery of cells obtained with the 2997 was decreased moderately. The percent of infused neutrophils localizing to the skin chamber was 0.101 and 0.044 for CS-3000 and 2997 cells, respectively; neither value was significantly different from normal (0.066). Thus, neutrophils collected using either of these cell separators function normally or nearly so in vivo.