CCL2, also called monocyte chemotactic protein-1 (MCP-1), is a member of the C-C or beta chemokine family that is best known as a chemotactic agent for mononuclear cells. Fibroblasts, glioma cells, smooth muscle cells, endothelial cells, lymphocytes and mononuclear phagocytes can produce CCL2 either constitutively or upon mitogenic stimulation, but monocytes and macrophages appear to be the major source. In addition to its chemotactic activity, CCL2 induces enzyme and cytokine release by monocytes, NK cells and lymphocytes, and histamine release by basophils that express its receptor CCR2. Additionally, it promotes Th2 polarization in CD4+ T cells. CCL2-mediated recruitment of monocytes to sites of inflammation is proposed to play a role in the pathology of atherosclerosis, multiple sclerosis and allergic asthma.