The interactions of a chondroitin sulfate-dermatan sulfate proteoglycan with serum very low (VLDL), low (LDL), and high density (HDL3) lipoproteins were studied with special reference to the nature of the interaction of LDL and the proteoglycan. The proteoglycan formed insoluble complexes with VLDL and LDL, but no complex was formed with HDL3. The proteoglycan (40 micrograms/ml) converted 98% of added LDL (150 micrograms of cholesterol/ml) into insoluble complex at a Ca2+ concentration of 30 mM. Physiologic concentrations of albumin inhibited insoluble complex formation with VLDL and LDL by 61.5 and 40.7%, respectively. The proteoglycan formed soluble complexes with LDL even in the absence of Ca2+. Optimum soluble complex formation occurred when the medium contained 66.6 micrograms of proteoglycan and 166.6 micrograms of LDL cholesterol/ml. Specific modifications of the lysine and/or arginine residues of LDL prevented complex formation with the proteoglycan, thus indicating the requirement of the positive charges of the protein moiety of LDL in the interaction. Removal of the protein core or the glycosaminoglycan chains of the proteoglycan prevented interaction with LDL. Desulfation of the proteoglycan molecule also inhibited complex formation with LDL. Thus, the native state of the arterial proteoglycan molecule confers optimum charge density for specific interaction with serum apoprotein B-containing lipoproteins.