The surface force technique was employed to investigate the adsorption of positively charged lysozyme onto negatively charged mica surfaces in 10(-3)M NaCl at pH 5.6 at lysozyme concentrations ranging from 0.002 to 0.2 mg/mL. At equilibrium the adsorbed lysozyme nearly neutralizes the surface charge of the mica at all bulk lysozyme concentrations investigated. Prior to charge neutralization the decay length of the long-range force is consistent with the electrostatic double-layer force predicted by the DLVO theory. At low concentration, 0.002 mg/mL, a densely packed side-on oriented layer adsorbs on the mica surface. Above 0.02 mg of lysozyme/mL, a rather thick layer is adsorbed onto the surface. It consists of an inner, strongly bound layer of both side-on and end-on adsorbed proteins and an outer layer of weakly adsorbed proteins. An adhesion force is established upon contact of the adsorbed protein layers. The force measured between one lysozyme-coated surface and one bare mica surface is attractive at short separations. It was demonstrated that, at a concentration of 0.02 mg/mL, lysozyme adsorbs "irreversibly" with respect to dilution with 10(-3) M NaCl.