The infiltration of monocytes into the lesioned site is a key event in the inflammatory response after spinal cord injury (SCI). We hypothesized that the molecular events governing the infiltration of monocytes into the injured cord involve cooperativity between the upregulation of the chemoattractant stromal cell-derived factor-1 (SDF-1)/CXCL12 in the injured cord and matrix metalloproteinase-9 (MMP-9/gelatinase B), expressed by infiltrating monocytes. SDF-1 and its receptor CXCR4 mRNAs were upregulated in the injured cord, while macrophages immunoexpressed CXCR4. When mice, transplanted with bone marrow cells from green fluorescent protein (GFP) transgenic mice, were subjected to SCI, GFP+ monocytes infiltrated the cord and displayed gelatinolytic activity. In vitro studies confirmed that SDF-1α, acting through CXCR4, expressed on bone marrow-derived macrophages, upregulated MMP-9 and stimulated MMP-9-dependent transmigration across endothelial cell monolayers by 2.6-fold. There was a reduction in F4/80+ macrophages in spinal cord-injured MMP-9 knock-out mice (by 36%) or wild-type mice, treated with the broad-spectrum MMP inhibitor GM6001 (by 30%). Mice were adoptively transferred with myeloid cells and treated with the MMP-9/-2 inhibitor SB-3CT, the CXCR4 antagonist AMD3100, or a combination of both drugs. While either drug resulted in a 28-30% reduction of infiltrated myeloid cells, the combined treatment resulted in a 45% reduction, suggesting that SDF-1 and MMP-9 function independently to promote the trafficking of myeloid cells into the injured cord. Collectively, these observations suggest a synergistic partnership between MMP-9 and SDF-1 in facilitating transmigration of monocytes into the injured spinal cord.