The effects of interleukin-4 (IL-4) on amnion prostaglandin (PG) production have been determined. Human amnion cells from term placentae were grown to confluence and incubated with IL-4 either alone or with various concentrations of epidermal growth factor (EGF) of ionomycin. PGE2 production was determined using a specific radioimmunoassay. IL-4 alone stimulated PGE2 production in amnion cells in a concentration-related manner. IL-4 significantly enhanced the stimulatory actions of ionomycin and EGF on amnion cell PGE2 production. Ionomycin and IL-4 acted synergistically in their effects on PGE2 production by amnion cells whereas EGF and IL-4 acted more additively in this respect. We suggest that IL-4 production by immune effector cells in gestational tissues may contribute to the mechanisms of labor at term and preterm.