Digestion of chick-embryo cartilage proteoglycan (type H) with chondroitin AC II lyase or keratanase, in the presence of EDTA, N-ethylmaleimide, phenylmethanesulphonyl fluoride and pepstatin, resulted in the removal of the bulk of the chondroitin sulphate or keratan sulphate chains respectively, without altering the protein portion of the macromolecule. An exhaustive treatment of the proteoglycan with chondroitin AC II lyase followed by digestion with keratanase yielded a core fraction having the enzymically modified linkage oligosaccharides. Zonal sedimentation of this core preparation on a sucrose gradient in 0.5% SDS resulted in a single narrow band with a sedimentation coefficient of 6S. In 4 M-guanidinium chloride, the core preparation showed a tendency to aggregate to multiple-molecular-weight forms which could dissociate in the presence of Triton X-100. The results indicate that the preponderance of glycosaminoglycans in the proteoglycan molecule is a main reason for both polydispersity and hydrophilicity of the proteoglycan preparation, and further suggest that the enzymic procedures could prove useful as a method to obtain new information about the structure and properties of proteoglycan core molecules.