A potentially important tumor-host interaction is increased tumor-cell invasiveness in response to motility factors derived from stromal and lymphoid cells. Conditioned medium of IL-2-stimulated lymphocytes and fractions enriched in either T cells, natural killer (NK) cells, or monocytes induced motility in MCF-7 breast carcinoma cells. ELISA and antibody neutralization studies demonstrated that this effect was due to tumor necrosis factor-a (TNF-a) secretion by the lymphoid cells or the enriched fractions. Unstimulated leukocytes in direct contact with MCF-7 cells also induced motility that was inhibited by anti-TNF-a antiserum. Time-lapse video microscopy of cells exposed to 10 ng/ml TNF-a showed that motility was independent of its toxic effects. Immunoperoxidase showed that MCF-7 cells expressed both the 55-kDa and the 75-kDa TNF-a receptors (TNFR). Antiserum against the 55-kDa TNFR, like TNF-a, induced motility in MCF-7 cells. This was most likely due to cross-linking of the 55-kDa TNFR monomers, since the monomeric F(ab) did not produce this effect. Our results raise the possibility that TNF-ainduced motility is one mechanism by which tumor cells overcome the potential anti-tumor immune function of lymphocytes and macrophages in peri-tumoral infiltrates.