The aim of the present investigation is to study the effect of amniotic fluid myoglobin on prostaglandin synthesis by the fetal membranes. The paper presents results of radioimmunologic studies of myoglobin levels in various biological fluids during pregnancy: amniotic fluid, maternal serum samples obtained from an antecubital vein and the cervix, and neonatal urine. Amniotic fluid myoglobin levels were found to be significantly higher at term. Our results suggest that amniotic fluid myoglobin is likely to be derived from the fetus and excreted into amniotic fluid through fetal urine. Incubation of the fetal membrane with amniotic fluid, neonatal urine, and commercially available myoglobin enhanced phospholipase A2 activity, as well as arachidonic acid and prostaglandin F2 alpha production was compared to incubation with normal saline. The addition of anti-myoglobin antibodies to incubation media was found to block these effects. The results of our study show that myoglobin activates prostaglandin synthesis by the fetal membranes and stimulates the onset of labor.