Bone has high levels of two proteins which contain the vitamin K-dependent Ca2+ binding amino acid, gamma-carboxyglutamic acid (Gla). Bone Gla protein (BGP, osteocalcin) is a 49 residue water soluble protein and matrix Gla protein (MGP) is a 79 residue water insoluble protein. BGP is synthesized only by calcified tissues while MGP is synthesized by calcified tissues, cartilage, and all soft tissues tested. The synthesis of both proteins in osteoblastic cells is stimulated by 1,25(OH)2D3. Treatment of rats with the vitamin K antagonist Warfarin causes secretion of a non-gamma-carboxylated BGP which cannot bind to hydroxyapatite. Warfarin treatment reduces bone levels of BGP to 2% of normal, but does not appear to affect the structure of bone. The only abnormality seen in rats treated with Warfarin is the mineralization of several cartilages. The pattern of cartilage calcification is similar to that seen in the fetal Warfarin syndrome in humans, and may be due to abnormal synthesis of MGP.