FIBRINOLYTIC activity in the blood markedly decreases in the later months of pregnancy and in labour, but rapidly returns to normal after delivery1–3. Brakman and Astrup4 observed a significant increase in the selective inhibition of fibrinolysis induced by urokinase in plasma and serum of pregnant women, and their findings suggested that an inhibitory mechanism was involved in depressing fibrinolysis during pregnancy. The same characteristics of fibrinolytic inhibition in saline extract of human placenta have been reported by one of us5, that is placental extract inhibited only urokinase but not streptokinase or glycerol-activated plasmin. Because the placental extract contained blood components, however, it was uncertain whether the inhibition was derived from pregnancy blood or from placenta itself.