The AFM (atomic force microscopy) requires a stable and flat substrate on which a biological sample is readily immobilized. For instance, membrane protein 2D crystals in lipid bilayers have been adsorbed to silanized glass with covalently attachment using a photocrosslinker or to mica by physisorption under appropriate ionic conditions. For the immobilization of 2D crystals of water soluble proteins formed on lipid monolayers, a different approach is required to attach the highly hydrophobic tails of the lipid molecules. To find a suitably hydrophobic substrate, the hydrophobicity of different materials was determined by the sitting drop method. Briefly, a 10 μ1 droplet of deionized and filtered water was deposited on the substrate surface and the diameter measured. Table 1 shows a decrease in droplet diameter with increasing hydrophobicity, indicating that substrates such as silanized glass and HOPG (highly ordered pyrolytic graphite) may be suitable for adsorbing lipid monolayers.