The human CCL2 chemokine is implicated in many chronic inflammatory conditions. In the mouse, there are two CCL2 homologues, CCL2 (MCP-1/JE) and CCL12 (MCP-5). Both are potent monocyte chemoattractants and bind to and activate the same receptor, CCR2. The overlapping activities of these chemokines complicate the design of mouse model studies that are intended to mimic human disease. To study the roles of CCL2 and CCL12, we generated neutralizing antibodies specific to each chemokine. Consistent with binding and affinity analyses, the antibodies specifically inhibited CCL2-or CCL12-mediated Ca 2+ mobilization in THP-1 cells. When tested in nude mice bearing human PANC-1 pancreatic tumor cells in Matrigel TM plugs, CCL2 and CCL12 antibodies potently inhibited tumor angiogenesis, indicating that both CCL2 and CCL12 may contribute to tumor angiogenesis.