Marrow localization of neutrophils following circulation decreases with maturation and activation of the cells and is diminished by CXCR4 blockade. The effects of CXCR4 blockade on marrow localization of relatively immature, mature, and activated neutrophils were examined by treating mice with 40 to 80 μg/mouse blocking antibody (▪) or whole immunoglobulin (control; □) intravenously 30 minutes before infusing indium-labeled cells. Localization was then assayed 4 hours after cell infusion. Source of neutrophil population is on the abscissa. Data points are the means of 3 to 7 separate experiments (± SEM). Significantly different when compared with control-treated animals, ^*P < .001, ^**P < .05.