The effect of sodium dodecyl sulphate (SDS) on pre-adsorbed lysozyme layers on mica was investigated using a surface force technique and ESCA. Lysozyme was adsorbed onto mica surfaces from a 0.2 mg/ml protein solution containing 1 mM NaCl at pH 5.6 which nearly neutralizes the surface charge on the mica, and it was found that the adsorption was irreversible with respect to dilution with 1 mM NaCl solution. Addition of SDS (0.83 to 4 mM) to the bulk solution does not induce large-scale desorption of the adsorbed protein layers, the layer thickness remains the same, 4.8±0.4 nm, up to 4 mM. However, addition of SDS results in a pronounced gradual increase in the interfacial charge manifested in the long-ranged double-layer repulsion. At 6 mM the surfactant binding to the protein layer does change the layer properties. An initial surfactant induced denaturation of the adsorbed protein occurs, whereby the thickness of the layers decreases to 2.7±0.3 nm, yet the interfacial charge remains high, similar in magnitude to the charge of mica. When SDS is introduced at the cmc the protein layers are completely desorbed, we thus conclude that the desorption results from the combined effect of protein denaturation, and the increased complex charge.