Proteoglycan monomer (D1) and aggregate (A1) preparations were isolated from 4 M guanidinium chloride extracts of the Swarm rat chondrosarcoma. When EDTA, 6-aminohexanoic acid, and benzamidine were present in the solutions, the D1 preparation contained a single component (SO = 23 S), and the A1 preparation contained 30% monomer (SO = 23 S) and 70 percent aggregate (SO = 111 S). In the absence of EDTA, 6-aminohexanoic acid, and benzamidine, the A1 preparations contained only small proteoglycan fragments, indicating that extensive enzymatic degradation had occurred. The composition of the proteoglycan monomer was different from that of proteoglycan monomer preparations from normal hyaline cartilages in that it did not contain keratan sulfate and chondroitin 6-sulfate; only chondroitin 4-sulfate was found. The A1 preparation from the chondrosarcoma contained only one link protein, which was like the smaller (molecular weight of 40,000) of the two link proteins present in A1 preparations from bovine nasal cartilage. When the A1 preparation from the chondrosarcoma was treated with chondroitinase ABC and trypsin and the digest was chromatographed on Sepharose 2B, a complex was isolated which contained the link protein and the segments of the protein core from the hyaluronic acid-binding region of the proteoglycan molecules.