By means of a convenient automated method, the placental isoenzyme of alkaline phosphatase in pregnancy serum has been measured in 162 normal pregnancies. The basis of this measurement is, first, the heat inactivation of the nonplacental isoenzyme, and then the measurement of activity (substrate; phenylphosphate) under conditions of pH and substrate concentration which favor the action of the placental isoenzyme. Conditions for a manual method for measuring the placental isoenzyme are also given. The serum isoenzyme level of alkaline phosphatase observed an exponential course as a function of gestation time. The highest level coincided with delivery in half of the patients and in the others, it preceded delivery. On the basis of these normal data, it is now possible to recognize a deviation from the normal curve. The possible significance of the serum placental isoenzyme of alkaline phosphatase is discussed.