We report here that breast cancer cells from spontaneous tumors that arise in rat neu transgenic mice produce several chemokines capable of acting upon cells of the immune system. Moreover, mice bearing these spontaneous tumors possess splenic T cells as well as CD11c+, CD11b+ and CD19+ cells with an altered sensitivity to recombinant chemokines compared to naïve mice. A comparison between T-cell migration and the level of chemokines produced by the tumor cells revealed that the altered chemotactic activity was not a direct consequence of tumor-derived chemokines. These data suggest that a growing tumor may indirectly alter leukocyte chemotactic activity.