Alkaline phosphatase (ALP) - an enzyme of the hydrolase class (orthophosphonomonesterphosphohydrolase), basically consists of three isoenzymatic forms (hepatobiliary, bone, intestinal), during pregnancy appears a transitional placental form. Alkaline phosphatase catalyzes the cleavage of phosphoric acid from its organic compounds, the enzyme is located on the cell membrane and takes part in phosphorus transport. Alkaline phosphatase is involved in the processes associated with bone growth, so its activity in the serum of children is higher than in adults. The pathological increase in serum alkaline phosphatase activity is associated mainly with bone diseases (bone formation) and liver (obstruction of the bile ducts). In premature infants, children in the period of active growth, pregnant women (third trimester) may have increased physiological activity of alkaline phosphatase. To make an objective assessment of the hepatic or non-hepatic origin of the increase in alkaline phosphatase levels, it is necessary to use clinical data, but there are also biochemical methods that can differentiate between isoenzymes.