Tapping mode atomic force microscopy was used to study aggregation patterns of human immunoglobulin G (IgG) adsorbed from aqueous solutions onto silicon surfaces methylated in dichlorodimethylsilane. Formation of dendrite-like aggregates of IgG were observed after air exposure and appeared to be highly dependent on surface methylation as well as on protein concentration. Crosslinking the protein layer in 0.5% glutaraldehyde prior to air exposure resulted in homogeneous protein distributions with significantly reduced aggregation. This, and experiments using lyophilization techniques, indicates that aggregates form due to a lateral reorganization of the adsorbed protein layer at the air-liquid interface.