The bacterial growth-inhibiting activity of amniotic fluid from patients with polyhydramnios was compared with that of inhibitory and noninhibitory fluid from healthy women. Although it was postulated that an altered solute concentration in polyhydramnios may interfere with expression of bacterial inhibition, fluids from polyhydramnios cases were fully inhibitory. The inhibitory property in amniotic fluid from polyhydramnios cases was similar to previously described characteristics of a phosphate-sensitive inhibitor.