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.