Intact human fetal membranes (amnion, chorion and decidua) were incubated with 125I-labelled cytokines added to the fetal or maternal sides of the membrane. The transfer of 125I-labelled interleukin-6 (IL-6), 125I-labelled tumour necrosis factor alpha (TNF-alpha), 125I-labelled interleukin-1 alpha (IL-1 alpha) and 125I-labelled interleukin-1 beta (IL-1 beta) was determined by measurement of radioactivity in a gamma counter and the integrity of the cytokines was assessed by acid precipitation and by radioimmunoassay. IL-1 alpha and IL-1 beta were transferred through human fetal membranes in both feto-maternal and materno-fetal directions at similar rates. Only 2-4% of the cytokine originally added appeared to be intact on the opposing side of the membrane after 24 h of culture. Transfer of intact TNF-alpha (5-7%) and IL-6 (8-17%) was greater than that of the IL-1 isomers. Low but variable amounts of the four cytokines tested may be transferred through the human fetal membrane. This finding suggested that concentrations of cytokines in amniotic fluid would not reflect those produced by decidua if the fetal membranes are intact.