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H ematopoietic progenitor cells (HPCs) normally reside in the bone marrow (BM) but can be mobilized into the peripheral blood (PB) after treatment with GCSF or chemotherapy. In previous studies, we showed that granulocyte precursors accumulate in the BM during mobilization induced by either GCSF or cyclophosphamide (CY), leading to the accumulation of active neutrophil proteases in this tissue. We now report that mobilization of HPCs by GCSF coincides in vivo with the cleavage of the N-terminus of the chemokine receptor CXCR4 on HPCs resident in the BM and mobilized into the PB. This cleavage of CXCR4 on mobilized HPCs results in the loss of chemotaxis in response to the CXCR4 ligand, the chemokine stromal cell–derived factor-1 (SDF-1/CXCL12). Furthermore, the concentration of SDF-1 decreased in vivo in the BM of mobilized mice, and this decrease coincided with the accumulation of serine proteases able to directly cleave and inactivate SDF-1. Since both SDF-1 and its receptor, CXCR4, are essential for the homing and retention of HPCs in the BM, the proteolytic degradation of SDF-1, together with that of CXCR4, could represent a critical step leading to the mobilization of HPCs into the PB in response to GCSF or CY.