We have obtained overlapping cDNA clones for the entire coding sequence of the rat cartilage proteoglycan core protein from the Swarm rat chondrosarcoma. These cDNAs hybridize to two sizes of RNA transcripts of 8.2 and 8.9 kilobase pairs, which contain large 3'-untranslated sequences. The total contiguous cDNA is 6.55 kilobase pairs in size, and codes for a 2124-residue protein, including a 19-residue signal peptide. The sequence forms a series of eight structural domains including two globules, (Mr = 37,000 and 22,000) at the NH2 terminus of the molecule, one a complete and one a partial copy of the cartilage link protein. The major feature of the deduced protein sequence is a 1,104-residue segment containing 117 Ser-Gly sequences, the presumed chondroitin sulfate attachment sites. These are arranged in three domains of 428, 503, and 173 amino acids. The first domain contains 11 complete or partial repeats of a 40-residue unit, and the second domain is composed of six copies of a 100-residue repeating sequence. The first pattern is the more highly conserved, and may have given rise to the second. The carboxyl-terminal domain is a third globule which has homology with animal lectins.