1. A sequential extraction procedure using isotonic KCl and 4.0 M guanidine hydrochloride was used to solubilise proteoglycans from adult human costal cartilage under conditions minimising autolysis. Up to 28% of the total tissue hexuronate was extracted. 2. Proteoglycan fractions were prepared from the extracts by CsCl equilibrium density gradient centrifugation. 3. Each fraction exhibited distinct electrophoretic heterogeneity. 4. Proteoglycans extracted by isotonic solutions are relatively small and may be in vivo degradation products of whole molecules. The major fraction (A1) from high ionic strength extracts has a composition similar to that of A1 proteoglycans from adult human articular cartilage. A2 fractions differ and are highly enriched in protein. 5. A1 and A2 fractions from high ionic strength extracts contain proteoglycan aggregates, but to a much lesser extent than found in other cartilages like bovine nasal or porcine epiglottal cartilage. 6. The aggregates can be dissociated and a 'subunit" proteoglycan isolated by CsCl density gradient centrifugation in 4.0 M guanidine hydrochloride. 'Subunits' can reaggregate partially, when mixed with fractions of lower density from the gradient. 7. Addition of hyaluronic acid to A1GuHCl promotes a large increase in the amount of aggregate present. 8. The hyaluronic acid content of costal cartilage is not deficient compared to that of other hyaline cartilages. 9. Polypeptides with molecular weights suggesting their identity as 'link proteins' are present in A1 and A2 fractions. 10. Additional polypeptides of higher molecular weight than link proteins are also present in A1 and A2 fractions. They may represent the hyaluronic acid binding region of proteoglycans and compete with 'whole' proteoglycans for hyaluronate in the tissue.