The CXCR2 chemokine KC induces neutrophil release from marrow and this is significantly augmented by low doses of CXCR4 neutralizing antibody. To measure the mobilization of marrow-sequestered neutrophils by the CXC chemokine KC and the synergistic effect of low-level CXCR4 blockade 2 experiments were performed. (A) Mice were treated with KC, 2 μg intravenously (♦) or buffer control (□) 4 hours after labeled marrow neutrophils were sequestered to the marrow, as described. Subsequent marrow content of labeled neutrophils was assayed at 30, 60, and 120 minutes after chemokine infusion. Results are expressed as percentage decrease compared with control-treated animals. (B) Mice were treated with low-dose CXCR4 blocking antibody (0.02 μg intravenously), KC (2 μg intravenously), both antibody and KC, or buffer control 4 hours after labeled marrow neutrophils were sequestered to the marrow, as described. Subsequent marrow content of labeled neutrophils was assayed 60 minutes after chemokine/antibody infusion. Results are expressed as percentage decrease compared with control-treated animals. Data points are the means of 3 to 5 separate experiments (± SEM). ^*Significantly different when compared with control-treated animals, P = .02. ^**Significantly different when compared with control, KC, or low-dose blocking antibody alone, P < .05; ns indicates not significant.