Hematopoietic progenitor cells (HPCs) traffic to and are retained in the marrow through the trophic effects of the chemokine stromal cell-derived factor-1␣ (SDF-1␣) binding to its receptor, CXC chemokine receptor 4 (CXCR4). AMD3100 reversibly inhibits SDF-1␣/CXCR4 binding, and AMD3100 administration mobilizes CD34 ؉ cells into the circulation. We therefore tested the hypotheses that the combination of AMD3100 plus granulocyte colony-stimulating factor (G- CSF