The regulatory signals responsible for the increased biosynthesis of prostaglandins during parturition have not been established. Because interleukin-1 is capable of stimulating prostaglandin production by intrauterine tissues and is an inflammatory mediator, we propose that interleukin-1 may act as a signal for the onset of human labor in the setting of intrauterine infection. The purpose of these studies was to determine interleukin-1 activity in amniotic fluid and to establish its relationship with the onset of term and preterm labor. Amniotic fluid from 182 patients was assayed for interleukin-1 activity. Cell-associated interleukin-1 activity was detected in fluid obtained in the third trimester but not in fluid obtained in the second trimester of pregnancy, suggesting a maturational event in interleukin-1 production. The factor responsible for interleukin-1 activity had biochemical characteristics of interleukin-1α (estimated molecular weight of 14 kilodaltons, isoelectric point = 4.9), and its activity was blocked with an anti-interleukin-1α antisera. Women in spontaneous labor at term were more likely to have fluid phase interleukin-1 activity in amniotic fluid than women who were not in labor at term. Preterm labor in the setting of intraamniotic infection was associated with significant interleukin-1 activity in amniotic fluid. This bioactivity was predominantly attributable to interleukin-1β. A strong correlation between interleukin-1 and amniotic fluid concentrations of prostaglandin E2 and prostaglandin F2α was found in women in preterm labor. These findings support the hypothesis that interleukin-1 may play a role in the initiation of preterm labor associated with intraamniotic infection.