The regulatory signals responsible for the increased biosynthesis of prostaglandins during parturition have not been established. Interleukin-1 (IL-1) is capable of stimulating prostaglandin production by intrauterine tissues and is an inflammation mediator. It has been postulated as a signal for the onset of labor in the setting of intrauterine infection. A study was designed to determine if spontaneous labor at term was associated with changes in IL-1 activity in amniotic fluid. Such fluid was retrieved from 41 women in labor and from 39 women who were not in labor at term. Immunodetectable IL-1 beta was present in 22 of the 41 women in labor but in only 8 of the 39 women without labor. IL-1-like bioactivity was not different between the two groups at a dilution of 1:4, but at dilutions of 1:12, 1:36 and 1:108, amniotic fluid from women in labor had significantly higher bioactivity than that from women not in labor. A significant correlation was found between the bioassay and immunoassay results. Our data show that inhibitors of IL-1 bioactivity are present in amniotic fluid and suggest that in a subset of laboring women at term, an inflammatory reaction may play a role in triggering the onset of parturition.