The mechanical properties (critical stress intensity factor, hardness and Young's modulus) of 4 crystalline materials (two proteins. lysozyme and glucose isomerase and two non-proteins, glutamic acid and potassium sulphate) were measured with an indentation technique. It was found that the mechanical properties of lysozyme crystals depend on their state - dried, partly dried and moisture saturated - and their surroundings. The hardness, Young's modulus and the critical stress intensity factor of lysozyme crystals were observed to be much lower than those for the tested non-proteins, leading to the conclusion that crystalline lysozyme is comparalively more fragile and softer. In combination the mechanical properties of lysozyme and the non-proteins indicated that these materials were fairly brittle. Mechanical properties for crystals of the other protein, glucose isomerase, could not be quantified by indentation. However, qualitatively crystalline glucose isomerase was found to be more ductile and less fragile than crystalline lysozyme. The experimental findings were interpreted in terms of relative susceptibility to attrition and secondary nucleation in stirred industrial crystallizers.