The relation between the ABO blood groups and the serum level of the heat-stable placental alkaline phosphatase was studied in a series of about 400 pregnant women. The claim by other investigators that significantly lower enzyme levels are found in mothers of blood group 0 could not be confirmed in this investigation. No association was found between the ABO blood groups and the heat-sensitive (liver) alkaline phosphatase. Thus conclusive evidence for a blood group association has so far been established for intestinal alkaline phosphatase only.