Atomic force microscopy was used to study the structure of two membrane-associated proteins adsorbed to various supported phospholipid bilayers in physiological buffer. The aim was (a) to develop a preparation for the investigation of membrane-associated proteins at high resolution under native conditions and (b) to obtain information about the factors that determine the adsorption process and the structure of adsorbed proteins. Therefore, solid-supported membranes were formed on mica by spontaneous vesicle adsorption and spreading. Once a homogeneous, pinhole-free bilayer was formed, solutions containing the proteins at appropriate concentrations were applied. The two positively charged proteins chosen were myelin basic protein (MBP), which plays an essential role in the formation of functional myelin, and cytochrome c. On charged bilayers, MBP applied at concentrations of 0.5-50 mu g/mL formed aggregates of defined height (1.9 +/- 0.2 nm on negatively and 2.7 +/- 0.2 nm on positively charged lipids), which at high concentration covered the entire bilayer. These aggregates are probably monomolecular layers of MBP. On neutral lipid adsorbed MBP formed irregular aggregates. Cytochrome c showed a different adsorption: On negatively charged lipid. it formed aggregates of defined, monomolecular height (3.3 +/- 0.2 nm). On neutral bilayers small aggregates were observed. On positively charged lipid no adsorption was observed at all. These results indicate that (a) the adsorption of cytomchrome c can be interpreted in terms of a dominating electrostatic interaction; (b) MBP adsorption to lipid bilayers is not exclusively electrostatically driven and depends on the specific lipid bilayer composition; (c) the structure of adsorbed aggregates indicates a strong protein-protein interaction.