The murine monocyte chemoattractant protein 1, JE/MCP-1, like its human counterpart monocyte chemotactic and activating factor (MCAF), attracts monocytes-macrophages to tumor tissues. In previous studies we reported that expression of the JE/MCP-1 gene in murine colon carcinoma cells reduced their tumorigenicity and suppressed their metastatic potential. We now demonstrate that the growth and metastasis of the renal adenocarcinoma cell line RENCA are reduced when it was admixed with syngeneic fibroblasts engineered to secrete the JE/MCP-1 cytokine before injection. Culture supernatants of JE/MCP-1-expressing cells plus lipopolysaccharide (LPS) synergistically activated tumoricidal properties in syngeneic macrophages against RENCA cells. This activity was blocked by anti-JE/MCP-1 antibody, indicating that JE/MCP-1 was involved in priming the macrophages to respond to LPS. Moreover, alveolar macrophages isolated shortly after iv injections of JE/MCP-1-transfected cells were cytotoxic to RENCA cells in vitro. Collectively, these data suggest that in addition to its chemotactic properties, JE/MCP-1 can synergize with bacterial endotoxins to activate macrophages, thus providing a rationale for the use of the JE/MCP-1 protein as a modality for treatment of metastasis.