A 68‐year‐old male with nephrotic syndrome due to diabetic glomerulosclerosis was found to have high serum alkaline phosphatase after albumin infusions. By examining several albumin preparations, certain products were found to contain as high as 200 Sigma units of alkaline phosphatase per ml. By disc electrophoresis and heat treatment (56 C for 30 min), all of the alkaline phosphatases in the albumin preparations were found to be heat stable and of placental origin. Since a high alkaline phosphatase activity in serum may mislead diagnosis, it is quite important to know whether the high level of the enzyme in patient's specimens is of exogenous or endogenous origin. It is recommended that sera obtained from patients who have been treated with albumin should be heat tested before determination of alkaline phosphatase. Most of the alkaline phosphatase activity of liver, intestine, and bone origins is inactivated by heat, whereas that of placental origin is not inactiva