Continuous monitoring of heat denaturation of a mixture of alkaline phosphatase isoenzymes at 60 degrees C and pH 7.5 permits the simultaneous direct identification and quantitation of three isoenzymes: the placental isoenzyme, the L-phenylalanine-sensitive intestinal isoenzyme, and the liver isoenzyme (hepatocytic). The isoenzyme that is principally of bone origin cannot be identified as such without the help of other diagnostic aids and the patient's medical history. All human tissues contain alkaline phosphatase, many organs more than one of the isoenzymes. Liver alkaline phosphatase, which constitutes 40-50% of normal serum alkaline phosphatase activity, was measured in the serum of persons with various liver diseases. Its activity exceeded normal in all types of liver disease; in 80% of cases this increase was accompanied by increased gamma-glutamyl-transferase activity, but the quantitative correlationship (r = 0.54) was not as good as expected if both enzymes come from the same source and are indices of liver dieases. Liver alkaline phosphatase activity increases in the blood early in liver disease, before most liver tests show abnormalities. The other major isoenzyme of normal serum probably represents a mixture of isoenzymes from bone and reticulo-endothelial and vascular tissues, which all contain the same "very heat-labile" alkaline phosphatase. Cord blood and children's sera contain mostly this very heat-labile isoenzyme.