Serum-alkaline phosphatase in man is mainly derived from bone or liver. It has been confirmed that alkaline phosphatase of skeletal origin is heat-labile. Normal individuals have mean heat-stable values of 30.7%. In children, healing fractures, and patients with osteitis fibrosa the alkaline phosphatase of bone origin is increased, and in these groups the mean thermostable values were 9.6, 21.8 and 18.7%, respectively. In these cases the decrease in thermostable values appear to be diagnostic even in the presence of a normal total alkaline-phosphatase value. From this study, thermostable alkaline phosphatase appears to be useful in the evaluation of osteitis fibrosa in patients with chronic renal failure both in cases with normal or elevated total alkaline-phosphatase values.