Interleukin-1 (IL-1), a cytokine predominantly produced by activated macrophages, has been shown to possess a wide range of biological functions as well as to play a role as an immune mediator. As it has been reported that homogenates of peritoneal macrophages contained substances which stimulated progesterone production by leutinized granulosa cells of mice, we examined whether IL-1could modulate the steroidogenic production by luteinized granulosa cells of mice, we examined whether IL-1 could modulate the steroidogenic functions of granulosa cells. Porcine granulosa cells obtained from medium-sized follicles were cultured with IL-1 in the presence or absence of LH, and the effects of IL-1 on LH-stimulated as well as basal progesterone production by these cells were examined. Contrary to our expectation, IL-1 markedly inhibited the LH-stimulated progesterone production in a dose-dependent manner. The morphological leutinization of the granulosa cells induced by LH was also inhibited by IL-1. In addition to effects on LH-induced differentiation, IL-1 exhibited an inhibitory effect on basal (unstimulated) progesterone production as well. The ovarian granulosa cell was identified as one of the possible targets for IL-1, and it was shown for the first time that IL-1 can modulate steroidogenic functions of mammalian cells in culture.