Proteoglycans were isolated from either grossly normal or atherosclerotic pigeon aortas after extraction with 4 M guanidlne hydrochloride and purification by ion-exchange and sizeexclusion chromatography. The small-size proteoglycans (Km, 0.4, on Sepharose CL-4B) from both normal and atherosclerotic tissue contained primarily a dermatan sulfate proteoglycan with an intact molecular size of 220-330 kd and a 45-kd core protein. In addition to the dermatan sulfate proteoglycan, the preparation contained a proteoglycan recognized by monoclonal antibody (MAb) 5-D-4, indicating the presence of sulfated poty-JV-acetyllactosamine sequences common to corneal and cartilage keratan sulfate. Electrophoresis on sodium dodecyl sulfate-polyacrylamide gel revealed a polydisperse proteoglycan of 60-150 kd that was recognized by MAb 5-D-4. Significantly greater immunoreactivity with MAb 5-D-4 was observed for atherosclerotic compared with normal artery. After endo-β-D-galactosidase treatment of the proteoglycan from atherosclerotic aorta, diminished MAb 5-D-4 reactivity observed by both Western blot analysis and enzyme-linked immunosorbent assay demonstrated that the material was keratan sulfate. £ndo-β-D-galactosidase treatment of the intact proteoglycan generated core proteins of 28 and 38 kd. These studies suggest the presence of one or more keratan sulfate proteoglycans in grossly normal and atherosclerotic arteries. Immunochemical data suggest that sulfation of the keratan sulfate proteoglycan may be greater in atherosclerotic aorta.