Thromboplastin activity in amniotic fluid from 39 patients pregnant in weeks 14–40 has been studied. Amniotic fluid was obtained by amniocentesis. All fluids were fractionated and the activity studied in both supernatant and sediment in a highly specific and sensitive test system. All samples tested showed a very high thromboplastin activity. No correlation to gestational age was found. Amniotic samples from 19 of the patients were incubated with endotoxin. Endotoxin did not further increase the procoagulant activity.