Interleukin 10 (IL-10) inhibits the production of a wide range of cytokines in various cell types. The purpose of this study was to determine whether the expression of the IL-10 gene can influence tumor growth and metastatic properties of human melanoma cells. The human melanoma cell line, A375P, which does not produce endogenous IL-10, was transfected with a hygromycin expression vector (control) or a vector containing full-length murine IL-10 cDNA. A375P parental cells, A375P-Hygro, and A375P-IL-10-positive cells were injected s.c. and i.v. into nude mice. A375P-IL-10 cells produced significantly slower growing s.c. tumors and fewer lung metastases than control cells. The tumorigenicity of the human melanoma A375SM and the murine melanoma B16-BL6 cells was also significantly inhibited when they were admixed with A375P-IL-10 but not with A375P-Hygro before s. c. injection into nude mice. The suppression of tumor growth and metastasis was directly correlated with a decrease in neovascularity determined by immunostaining with anti-factor VIII. Because tumor-associated macrophages are the major source of angiogenic molecules in melanoma, we used reverse transcription-PCR to demonstrate that IL-10 down-regulates the production of vascular endothelial growth factor, the most potent angiogenic factor in activated macrophages. Other factors involved in angiogenesis such as IL-1beta, tumor necrosis factor-alpha, IL-6, and the proteinase matrix metalloproteinase-9 were also inhibited in activated macrophages by supernatants from A375P-IL-10 cells. Collectively, these data suggest that the production of IL-10 by tumor cells inhibits macrophages-derived angiogenic factors, and hence, tumor growth and metastasis.