Blocking CXCR4 results in mobilization of neutrophils from the marrow. To determine the effects of CXCR4 blockade on marrow-sequestered neutrophils, mice were treated with CXCR4-blocking antibody (▪) or control immunoglobulin (□), 2 μg intravenously 4 hours after labeled marrow neutrophils were sequestered to the marrow, as described. Subsequent changes in neutrophil localization were assayed 2 hours after antibody infusion. Data points are the means of 3 to 7 separate experiments (± SEM). ^*Significantly different when compared with control treated animals, P = .01. ns indicates not significant.