Osteocalcin (OC) is a product of osteoblasts and accumulated in the extracellular matrix of bone. It has been recognized that serum OC is a marker of osteoblast activity, and the levels reflect the rate of bone formation. The present assay system was developed to assess the major circulating forms of intact and the large N-terminal fragments. OC binds to the crystal of hydroxyapatite, at least partly, through gamma-carboxylation of three residues. Increased rate of immature undercarboxylated osteocalcin, therefore, might display risks for osteoporotic fractures in clinical studies. However, at present, measurement of OC does not substitute for bone mass measurement and only provide limited values to evaluate the conditions of patients with primary osteoporosis.