Human serum osteocalcin is a well known bone formation marker. On the basis of their different affinities for hydroxyapatite, the total immunoreactive osteocalcin may be separated into two fractions. Six commercial test kits for osteocalcin were compared. All kits reacted with both osteocalcin fractions but the absolute amounts found in the same serum samples differed widely. During serum storage at room temperature, there was no significant loss of osteocalcin during the first 6 h. After longer storage periods, the recorded decrease of osteocalcin depended on the system used: with two kits, over 80% of the original immunoreactive antigen was left after 9 days. It is considered that the different osteocalcin fractions may become useful as markers for different metabolic bone processes. A more precise definition of the various circulating osteocalcin fractions, and the development of separate tests for each fraction, are requirements for the optimal use of osteocalcin as a diagnostic tool for metabolic bone disorders.