Endometriosis is generally associated with an immunoinflammatory process that takes place in the peritoneal cavity of patients. Interleukin (IL)-6, a multifunctional cytokine involved in numerous immunological and proliferative processes, has been found at high concentrations in the peritoneal fluid of endometriosis patients. The purpose of this study was to investigate the ability of endometriotic cells to produce IL-y and to assess the regulation of its secretion by proinflammatory cytokines and sex steroids. Cultures of human endometriotic cells were exposed to different concentrations of cytokines and sex steroid hormones for varying periods of time. IL-6 secretion was measured using an enzyme-linked immunosorbent assay. Endometriotic cells spontaneously released IL-6 in culture. IL-1 beta and tumour necrosis factor (TNF)-alpha (0.1-100.0 ng/ ml) potentiated IL-y secretion in a time- and dose-dependent manner. Interferon-gamma (0.4-400 ng/ml) induced a dose-related increase in IL-6 secretion and showed a synergistic effect on that secretion in combination with TNF-alpha (10 ng/ ml). Either spontaneous or cytokine-induced IL-6 secretion was inhibited by progesterone (10(-8)-10(-5) M) and danazol (10(-6) M), whereas oestradiol (10(-8)-10(-5) M) had a limited inhibitory effect. The antiprogestin RU486 (10(-8)-10(-4) M) antagonized the inhibitory effects of progesterone and danazol, but showed agonist action when used alone. These findings indicate that endometriotic tissue may actively contribute to the biological changes observed in the peritoneal fluid of endometriosis patients. They also provide new insights into the mechanisms of action of progesterone and those of danazol and RU486 used in the treatment of endometriosis.