Markers of bone formation determined in serum include alkaline phosphatase, bone-specific alkaline phosphatase, osteocalcin [bone γ-carboxyglutamic acid peptide (BGP)] and procollagen type I carboxyterminal propeptide. Recently, advances have been made in the immunoassary of bone-specific alkaline phosphatase. This is a marker for osteoblastic activity; it is very stable and is not primarily dependent on kidney function because it is degraded in the liver. BGP is not specific for bone formation because it increases in serum during bed rest (which involves increased bone resportion), and it is not stable. Furthermore, the elimination of BGP is dependent on glomerular filtration rate. Procollagen type I carboxyterminal propeptide is not as sensitive as bone-specific alkaline phosphatase because it increases less in women after the menopause. Urinary pyridinoline and deoxypyridinoline determined by high-performance liquid chromatography are regarded as the best methods for measuring bone resorption. These might be replaced by type I collagen crosslinked N-telopeptide or CrossLaps in the future in laboratories not equipped with a high-performance liquid chromatography system. Serum markers of bone resorption are currently under investigation. An immunoassay for the tartrate-resistant acid phosphatase in serum should be a very promising tool for the quantification of bone resorption.