Dispersed cells were prepared from amniotic membranes obtained either by caesarean section near term before labor (CS) or after spontaneous vaginal delivery (SL) and from human endometrial curettings. The cells were maintained separately in primary culture for about 18 h. Production rates (PR) of PGE and PGF during incubation for 1 h in defined medium were determined when the cell-types were separate (n = 60) or combined (n = 27) and when endometrial cells were incubated in medium conditioned by CS amnion cells (n = 13) or SL amnion cells (n = 12). The PR of PGE by CS amnion cells was five times greater than that of PGF and there was a two-fold increase (p less than 0.01) in PGE but not PGF by SL cells. Co-incubation was associated with a 25-32% fall in PR of both PGE and PGF (p less than 0.01) compared to the sum from separately incubated cells when CS cells were used whereas values for co-incubated SL cells did not differ from controls. Conditioned medium from CS but not SL cells inhibited PGE and PGF output by 30% and 40% (p less than 0.01) respectively. These findings suggest that human amnion cells release an inhibitor of prostaglandin synthesis in endometrial cells.