The pro-inflammatory chemokine, monocyte chemoattractant protein-1 (MCP-1), plays a fundamental role in monocyte recruitment and has been implicated as a contributing factor to atherosclerosis. The predominant cell types within the vessel wall--endothelial cells, smooth muscle cells, and macrophages--all contribute to overexpression of MCP-1 in atherosclerotic tissue. In this report we assess the role of MCP-1 expression by leukocytes on lesion progression in a murine model susceptible to atherosclerosis. Bone marrow cells from mice overexpressing a murine MCP-1 transgene on a background of apoE-deficiency or from control mice were transplanted into irradiated apoE-knockout mice. After repopulation of apoE-knockout mice with bone marrow containing the MCP-1 transgene, macrophages expressing the MCP-1 transgene were found in several tissues, including the aorta. Qualitative assessment of atherosclerosis in these mice revealed increased lipid staining, a 3-fold (P<0.001) increase in the amount of oxidized lipid, and increased immunostaining for macrophage cell surface markers with anti-F4/80 and anti-CD11b antibodies. There were no differences in plasma lipids, plasma lipoprotein profiles, or body weight between the 2 groups. These results provide the first direct evidence that MCP-1 expression by leukocytes, predominately macrophages, increases the progression of atherosclerosis by increasing both macrophage numbers and oxidized lipid accumulation.