Fibroblasts are a dominant cell type in most human solid tumors. The possibility that fibroblasts have the capacity to interact with and modulate the function of tumor-associated T lymphocytes makes them a potential therapeutic target. To address this question, primary cultures of fibroblasts derived from human lung tumors were established and cultured with T cells derived from the same tumor. The tumor fibroblasts significantly enhance the production of IFN-gamma and IL-17A by the tumor-associated T cells following a CD3/CD28-induced activation of the T cells. This enhancement was fibroblast cell dose-dependent and did not require direct contact between the two cell types. Tumor-associated fibroblast-conditioned media similarly enhanced both IFN-gamma and IL-17A in activated T cells, and this enhancement was significantly reduced by Abs to IL-6. Conditioned media derived from activated lymphocyte cultures significantly enhanced IL-6 production by tumor fibroblasts. A similar enhancement of IFN-gamma and IL-17A was observed when activated T cells from a normal donor were cultivated with skin fibroblasts derived from the same donor. These results establish that fibroblasts and autologous lymphocytes, whether derived from the tumor microenvironment or from nonmalignant tissues, have the capacity to reciprocally interact and modulate function. In contrast to other reports, fibroblasts are shown to have an immunostimulatory effect upon activated T lymphocytes. The ability of fibroblasts to enhance two T cell cytokines known to have an impact upon tumor progression suggests that fibroblasts play an important role in tumor pathogenesis that could be exploited therapeutically.