The preparation of a proteoglycan (PG) from human aortic intima-media is described. The PG was obtained from intima-media homogenates by differential centrifugation, exclusion chromatography and preparative agarose electrophoresis. Crude or purified preparations of the proteoglycan are capable of forming specific insoluble complexes with LDL, purified or in serum. This product has been labelled lipoprotein-complexing proteoglycan (LCP-3). On agarose and cellulose acetate electrophoresis LCP-3 appears as a single band. However, its glycosaminoglycan (GAG) moiety shows a composition and chromatographic behaviour compatible with hybrid or mixed chains of chondroitin-6-so4, dermatan sulfate and heparin and/or heparan sulfate. The specificity of LCP-3 for LDL disappears when it is treated with testicular hyaluronidase or proteolytic enzymes. Ionic strength, pH, Ca++ and Mg++ modulate the amount of LDL insolubilized. The amino acid composition of the protein from LCP-3 is that of a basic protein(s), perhaps bound covalently through xylose--serine residues to the GAG's. The estimated molecular weight of LCP-3 is 1 to 5 x 10(6) daltons. The presence of LCP-3 to intima-media and its specificity for interacting with LDL at conditions near to physiological ones are suggestive of the role that this type of structure may play in the association of the atherogenic lipoproteins with components of the arterial intima-media.