Skeletal alkaline phosphatase, an enzyme localised in the osteoblasts as well as in the extracellular layers of a newly synthesized matrix, is released into the circulation by an unclear mechanism. Serum alkaline phasphatase is derived from several sources including the liver, bone (where it is needed for osteoblastic activity), kidney, intestine, and placenta (in pregnant women). Therefore, elevated levels of serum alkaline phosphatase are observed in a variety of disorders such as rickets and osteomalacia, Paget's disease, hyperparathyroidism, during the healing of fractures, osteoblastic carcinoma and intra- and extrahepatic cholestasis. Serum total alkaline phosphatase activity is the most commonly used marker of bone formation, but it lacks sensitivity and specifity. Many assay procedures have been developed in attempts to improve the specifity and the sensitivity of serum alkaline phosphatase measurement. A new reagent which has recently been reported is likely to be available in the future.