Human blood plasma contains alkaline phosphatase (EC 3.1.3.1) isoenzymes from bone, liver and intestine. Blood of pregnant women in the third trimester of gestation also contains an isoenzyme of alkaline phosphatase from placenta, placental-AP (PLAP). Serum from pregnant women in the last trimester of gestation shows activity of soluble placental alkaline phosphatase (sol-PLAP). It is also known that a membrane bound high molecular weight placental-AP (high Mr-PLAP) is present in butanol extracts from placental tissue. Nevertheless, up to now, high Mr-PLAP has not been founded in human plasma. A method developed in this laboratory allows detection of membrane-bound alkaline phosphatase in pellet of plasma centrifuged at 100,000x g. By applying this method we have detected high molecular weight placental alkaline phosphatase in plasma of healthy pregnant women in the third trimester of pregnancy.