Cartilage matrix glycoprotein (CMGP) is a disulfide-bonded 550,000-dalton protein that is synthesized by chondrocytes and ciliary epithelial cells. We have purified the protein from bovine and porcine articular cartilage and have sequenced two peptides, which both have significant homology with human ceruloplasmin, a copper-binding oxidase. Immunolocation analysis indicates that a commercial polyclonal antiserum to human ceruloplasmin reacts with bovine cartilage CMGP. Chelating columns made with copper bind CMGP from bovine cartilage extracts. CMGP is present in bovine chondrocyte membrane preparations purified from sucrose density gradients. Oligonucleotide probes have been synthesized based on the published sequence of the 3'-untranslated region and a portion of the C terminus of human ceruloplasmin and have been used to amplify a cDNA fragment from bovine cartilage and human liver libraries. CMGP demonstrates oxidase activity towards p-phenylenediamine similar to that of ceruloplasmin. These studies suggest that CMGP is closely related to, if not identical with, ceruloplasmin. It is possible that CMGP may be involved in metal transport into and/or within the chondrocyte.