We have investigated the adsorption of lysozyme molecules on mica substrates in buffer by atomic force microscopy (AFM). The stability of the adsorbate is influenced by buffer conditions. At pH 4 single molecules can be imaged stably, while at pH 6.4 only aggregates of molecules are observed. Maps of the tip-sample interaction were obtained by recording laterally resolved force curves. These force curves showed that the adhesion of the tip on the lysozyme is smaller than on the mica. The intrinsic viscoelastic properties of lysozyme could be calculated by analyzing these force curves.