Subclinical intrauterine infections have been proposed to be one of the leading causes of preterm labor. The determination of inflammatory cytokines IL-1 beta and IL-6 in the amniotic fluid may be useful to identify women who have infection-associated preterm labor. Amniotic fluid was collected from 99 women during amniocentesis, during cesarean section or at the time of amniotomy using sterile technique. IL-1 beta and IL-6 were determined by a specific ELISA. Fluid of each sample was cultured for aerobic and anaerobic bacteria and for Mycoplasma hominis and Ureaplasma urealyticum. Different populations were identified according to the criteria "gestational age", "active labor", "positive amniotic fluid cultures". Interleukin-6 was detectable in all samples of amniotic fluid. The second-trimester (weeks 14-27) amniotic fluid concentration of IL-6 (18-2270 pg/ml) was higher than in the third trimester (weeks 28-42, 4-329 pg/ml). The difference was significant. Women in active labor had higher levels of IL-6 in their amniotic fluid than women not in labor (p < 0.01). There is no significant difference between women with preterm labor and delivery (weeks 28-37, 597-8670 pg/ml) and with term labor and delivery (weeks 38-42, 24-8300 pg/ml). Only culture negative samples were included in this population. Interleukin-1 beta was not detectable in the majority (90%) of these samples. 30% of the women in labor had positive amniotic fluid cultures. The IL-6 concentration of this population was not elevated in comparison to women in labor with negative cultures. Interleukin-1 beta was present in high concentrations (5-1100 pg/ml) in all fluid samples with detectable bacteria. Our data suggest that IL-1 beta may indicate subclinical uterine infection. IL-6 is elevated in all fluid samples of women in active labor.