Compelling evidence has emerged in recent years indicating that stromal cells play a critical role in disease progression. CXCR4 is a G-protein-coupled receptor with a major role in lymphocyte homing. Its ligand, CXCL12, is a highly efficient chemotactic factor for T cells, monocytes, pre-B cells, dendritic cells and myeloid bone marrow-derived cells (BMDCs). In addition, the CXCR4-CXCL12 axis plays a central role in tumor growth and metastasis. To evaluate the effect of genetic CXCR4 reduction on metastasis development, murine melanoma B16 cells were injected into the tail vein of C57BL/6 CXCR4(+/+) and CXCR4(+/-) mice in the presence of the CXCR4 inhibitor, Plerixafor (previously named AMD3100). Although lung metastases developed in wild-type CXCR4(+/+) and heterozygote CXCR4(+/-) mice, nodules were significantly smaller in the latter. CXCR4 pharmacological inhibition by Plerixafor further reduced lung metastases in CXCR4(+/-) mice, preserving the pulmonary architecture (4.18 ± 1.38 mm(2) vs. 1.11 ± 0.60 mm(2), p = 0.038). A reduction in LY6G-positive myeloid/granulocytic cells and in p38 MAPK activation was detected in lungs from CXCR4(+/-) mice compared to CXCR4(+/+) mice [LY6G-positive myeloid CXCR4(+/-) vs. CXCR4(+/+) (p = 0.0004); CXCR4(+/+) vs. CXCR4(+/+) Plerixafor-treated (p = 0.0031)] suggesting that CXCR4 reduction on myeloid-derived cells reduced their recruitment to the lung, consequently impairing lung metastases. Our findings argue in favor of a specific role of CXCR4 expressed in stromal cells that condition the pro-tumor microenvironment. In this scenario, CXCR4 antagonists will target neoplastic cells as well as the pro-tumor stromal microenvironment.