Proteoglycans were extracted from porcine lungs with 4 M guanidinium chloride. The extract was subjected to associative density gradient centrifugation, and four equal fractions, labeled A1 through A4 from the bottom to the top of the gradient, were obtained. The pooled A1 fractions containing proteoglycan aggregates were further fractionated by dissociative density gradient centrifugation to yield four equal fractions labeled A1D1 through A1D4 from the bottom to the top of the gradient. These fractions were analyzed for their protein, uronic acid, glucosamine, galactosamine, hexose, and sialic acid content. The fraction A1D1 with the highest buoyant density had the highest content of uronic acid and galactosamine, and lowest content of protein, indicating the enrichment of proteoglycan monomers at the bottom of the dissociative density gradient. As the density of the gradient decreased, the protein, hexoses, and sialic acid content increased, whereas uronic acid and galactosamine content decreased. The amino acid analysis showed similar composition for all four fractions with aspartic acid, serine, glutamic acid, proline, glycine, alanine, valine, and leucine as the major constituent amino acids. No hydroxyproline was detected in any of the fractions. As the buoyant density of the fractions decreased, the aspartic acid content increased and glycine content decreased.