Discs of human amnion prepared from nine women delivered by elective caesarean section at term were incubated with and without purified human amniotic fluid surfactant (9 μg lipid P/ml), and the output of prostaglandin E (PGE) was estimated by radioimmunoassay. Surfactant stimulated the release of PGE from 3·8 (SD 2·9) to 5·4 (SD 2·5) pmol/mg dry weight/3 h (P<0·01). Arachidonic acid accounted for 2% of the fatty acids of surfactant lecithin. These results show that surfactant has a stimulatory effect on amniotic PGE production and that it is an important source of arachidonic acid in amniotic fluid.