Cytokines locally delivered to the site of a tumor boost both specific and nonspecific host anti-tumor defenses. Interleukin (IL)-13 is a recently described cytokine produced by mouse type 2 helper T lymphocytes. The aim of this study was to evaluate the inhibition of tumor growth induced by IL-13 delivered locally within or around transplanted tumor cells in mice. We observed that local administration of IL-13 at the site of transplanted tumor cells in vivo had potent inhibitory effects on growth of both immunogenic (P815 mastocytoma, H-2d) or nonimmunogenic (3LL lung carcinoma, H-2b) tumor cells. Mice injected with transfected P815 cells secreting large amounts of IL-13 rejected the P815 tumor and developed systemic specific anti-tumor immunity leading to long-lasting specific anti-tumor protection. Less efficient anti-tumoral effects were obtained with the nonimmunogenic 3LL tumor model when local administration of IL-13 was achieved by co-inoculating xenogeneic chinese hamster ovary (CHO) IL-13 cells. Several local injections of CHO IL-13 cells were needed to obtain rejection of 3LL tumors and no induction of long-lasting anti-3LL memory was obtained. Several studies were performed to elucidate the IL-13 anti-tumoral effects. Experiments with nude mice indicated that Il-13 can also stimulate nonspecific anti-tumor defenses. The histological examination of P815 IL-13 cells undergoing rejection showed monocytic cells and neutrophils infiltrating the tumor. Studies indicated that IL-13 administered in vitro did not directly stimulate the cytotoxicity of peritoneal macrophages and natural killer cells. However, experiments with Boyden chemotaxis chambers indicated that IL-13 was chemotactic for macrophages. Finally, preliminary experiments in vitro suggest that IL-13 improved antigenic presentation of P815 membranes. Thus, anti-tumor effects of IL-13 in vivo most probably result from pleiotropic effects including recruitment of nonspecific cells and improved stimulation of immune-specific anti-tumor effectors.