The radioactively labelled constituents present in bone matrix were compared 12 days after injection of either [ 14 C]glucosamine or plasma [ 14 C]glycoprotein. Both precursors are utilized in the synthesis of organic matrix by bone tissue. Cortical bone from animals injected with [ 14 C]glucosamine contains radioactivity derived from glucosamine and plasma glycoproteins and all glycoprotein fractions are labelled. Plasma [ 14 C]glycoprotein labels the less acidic glycoproteins to a greater extent than the more acidic components. An antibody has been raised against the less-acidic-glycoprotein fraction of bone. The latter contains a glycoprotein of α-mobility that appears to be concentrated specifically in bone tissue and which is present also in plasma. This α-glycoprotein accounts for a large proportion of the components labelled and retained in bone matrix after [ 14 C]glucosamine injection.