The ability of des-gamma-carboxy bone Gla protein (dBGP) and des-gamma-carboxy matrix Gla protein (dMGP) to act as substrates for the rat liver vitamin K-dependent carboxylase has been investigated. An amino-terminal 'propeptide' is present on the intracellular form of BGP and is thought to interact with a recognition site on the enzyme. dBGP, lacking this extension, is a poor, high apparent Km, carboxylase substrate, but is a much better substrate when free propeptide is added. MGP lacks an amino-terminal propeptide, but contains a a homologous region in the mature protein. dMGP is an excellent substrate for the carboxylase with a low apparent Km and its carboxylation is inhibited by free propeptide.