Cytokine levels in amniotic fluid have been shown to increase towards term in normal pregnancies, and may play a regulatory role in parturition by stimulating the local production of prostaglandins. The work reported in the present paper was conducted in order to test the hypothesis that the increased cytokine levels may be induced by a subclinical inflammatory reaction in intrauterine tissues. The concentrations of tumor necrosis factor (TNF), interleukin 1 (IL-1), interleukin 2 (IL-2) and interleukin 6 (IL-6) were determined in samples of amniotic fluid from 38 women in delivery at term, after a clinically normal pregnancy. In 33 of the cases, tissue material was available for histological examination. In these, the extent of inflammatory cell infiltration was assessed in the fetal membranes, placenta and umbilical cord. A close interrelation was observed between the levels of the mediators typically released during inflammatory processes (TNF, IL-1, IL-6). Frank chorioamnionitis was not found in any of the histological specimens, although most placentae showed varying degrees of granulocyte infiltration in the fibrin layer under the chorion, sometimes also in the chorionic membrane. The degree of such leukocytic infiltration correlated positively with the levels of TNF, IL-1 and IL-6. These findings lend support to the hypothesis that a low-level inflammatory process may be a normal occurrence in the term placenta, and that this process may induce the production of cytokines, which, in turn, may play a role in the regulation of parturition. Such inflammation could be due to exposure of the fetal membranes to microbial material from the vagina, as the cervix dilates towards term.