Aggrecan is a bottlebrush shaped macromolecule found in the extracellular matrix of cartilage. The negatively charged glycosaminoglycan (GAG) chains attached to its protein backbone give aggrecan molecules a high charge density, which is essential for exerting high osmotic swelling pressure and resisting compression under external load. In solution, aggrecan assemblies are insensitive to the presence of calcium ions, and show distinct osmotic pressure versus concentration regimes. The aim of this study is to investigate the effect of ionic environment on the structure of aggrecan molecules adsorbed onto well-controlled mica surfaces. The conformation of the aggrecan was visualized using Atomic Force Microscopy. On positively charged APS mica the GAG chains of the aggrecan molecules are distinguishable, and their average dimensions are practically unaffected by the presence of salt ions. With increasing aggrecan concentration they form clusters, and at higher concentrations they form a continuous monolayer of conforming molecules. On negatively charged mica, the extent of aggrecan adsorption varies with salt composition. Understanding aggrecan adsorption onto a charged surface provides insight into its interactions with bone and implant surfaces in the biological milieu. (C) 2010 Wiley Periodicals, Inc.dagger J Polym Sci Part B: Polym Phys 48: 2575-2581, 2010