The interaction forces between a silica sphere and two-dimensional bacteriorhodopsin crystals (bR) were measured in NaNO3 solutions at different pH values, and at two electrolyte concentrations. Using the known zeta potential for the silica sphere as a function of pH, the surface potential of the bR crystals could be determined. The isoelectric point (iep) was found to be pH 4.3 +/- 0.5. The crystals had a uniform thickness on substrates of 5.2 +/- 0.5 nm, and the Hamaker constant for the bR-H2O-SiO2 system was estimated to be 0.5-1.0 x 10(-20) J at pH values near the iep. Analysis of the force curves suggested that the surface charge on the protein surface remained constant during approach of the silica colloid probe.