To clarify the role of interferon-gamma (IFN-gamma) in reproduction, we have examined its effects on cytokine production by human endometrial stromal cells (ESC). Concentrations of interleukin (IL)-6, IL-8, monocyte chemoattractant protein-1 (MCP-1), and macrophage colony-stimulating factor (M-CSF) in the culture media of normal ESC and an endometrial stromal sarcoma cell line, MaMi, were measured using an enzyme-linked immunosorbent assay. Both non-stimulated ESC and non-stimulated MaMi cells constitutively secrete IL-6, IL-8, MCP-1, and M-CSF. In a dose-dependent manner, IFN-gamma increased the concentrations of IL-6, MCP-1, and M-CSF and reduced the concentrations of IL-8 in ESC and MaMi cells. These results suggest that IFN-gamma produced by both decidual inflammatory cells and the developing embryo plays a role in the maintenance of early pregnancy by modulating the production of these cytokines by human ESC.