Proteoglycans were extracted, in a yield of about 90%, from costal cartilage of young, growing guinea‐pigs. Three solvents were used in sequence: 0.4 M guanidine · HCl, pH 5.8, 4 M guanidine · HCl, pH 5.8, and 4 M guanidine · HCl/0.1 M EDTA, pH 5.8. The proteoglycans were purified and fractionated by cesium chloride density gradient ultracentrifugation under associative and dissociative conditions. Gel chromatography on Sepharose 2 B of proteoglycan fractions from associative centrifugations showed the presence of both aggregated and monomer proteoglycans. The ratio of aggregates to monomers was higher in the second extract than in the other two extracts. Dissociative gradient centrifugation gave a similar distribution for proteoglycans from all three extracts. Thus, with decreasing buoyant density there were decreasing ratios of polysaccharide to protein, and of chondroitin sulfate to keratan sulfate. In addition, there was with decreasing density an increasing ratio of chondroitin 4‐sulfate to chondroitin 6‐sulfate. Amino acid analyses of dissociative fractions were in accordance with previously published results. On comparing proteoglycan monomers of the three extracts, signific
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