The forces between beta-casein layers adsorbed onto mica surfaces have been measured as a function of surface separation by using the interferometric surface force apparatus. Both hydrophilic pure mica and a mica surface, which has been made hydrophobic by Langmuir-Blodgett deposition of dimethyldiocta-decylammonium bromide, were used. A long-range repulsive force, most probably of electrostatic origin, was observed between beta-casein layers adsorbed on hydrobized mica. The results suggest that p-casein forms a monolayer on this surface, where the outer part is less densely packed and protrudes far out into the solution. This open brushlike structure can readily be compressed on which an attractive force arises. The portion of the monolayer closest to the hydrophobized surface is much more compact and has low compressibility. On the pure hydrophilic surface a bilayer structure is more likely, with the a compact inner layer and an outer layer which has a similar structure to the monolayer formed on a hydrophobic surface.