Tumor-associated chemokines, including CC chemokine ligand 2/monocyte chemoattractant protein-1 (CCL2), are thought to play many roles in cancer progression. Here we demonstrate the novel finding that during growth of the D1-7,12-dimethylbenzanthracene-3 mammary tumor in BALB/c mice, there is a dramatic up-regulation of CCL2 in splenic T cells at both the mRNA and protein levels upon stimulation. Of particular relevance is the finding that tumor-infiltrating T cells also produce high levels of CCL2. While a variety of tumor cell lines have been found to produce CCL2, we found no detectable levels of CCL2 protein in supernatants of the cultured mammary tumor cells. Investigation of the mechanisms involved in CCL2 induction showed that treatment of splenic T cells with the tumor-derived factors GM-CSF and phosphatidyl serine (PS) resulted in increased CCL2 production. This increased production may be involved in the downregulation of IFN-gamma by the T cells of tumor-bearing mice previously reported in this model, as treatment of splenic T lymphocytes with CCL2 resulted in a decreased secretion of IFN-gamma by those cells.