The mechanical properties of high-quality single crystals of tetragonal hen-egg-white lysozyme from hen egg white were investigated by the indentation method. The indentation marks were clearly observed on the crystal surface and elastic recovery did not occur. The micro-Vickers hardness of the crystals in the wet condition was observed to be 20 MPa at room temperature. To investigate deformation, the temperature dependence of the hardness was investigated from 278 to 308 K. The hardness strongly depended on the temperature above 290 K. From these results the deformation of lysozyme single crystals was explained in terms of dislocation mechanisms.