Bacterial growth inhibition in amniotic fluid is associated with a compound or class of compounds which resembles antibacterial cationic peptides. The inhibitor studied in amniotic fluid is sensitive to treatment with monobasic and dibasic potassium phosphate, and the inhibitory activity of amniotic fluid is lost following adsorption onto bentomite. The inhibitory component in amniotic fluid interacts with bacterial cells by adsorption or is internalized by viable cells only. The inhibitory quality of amniotic fluid is not destroyed by heating to 100 degrees C. for 15 minutes, and this heat-stable inhibitory activity is associated with the compound or compounds which are phosphate sensitive. A possible mode of antibacterial action which is consistent with the data presented is discussed.