An advanced technique for bulk protein crystallization based on solvent evaporation is introduced. Hen egg white lysozyme is crystallized from its buffered aqueous solution in a stirred tank at low pressure and, thus, moderate boiling temperatures. No crystallizing agent to reduce the solubility is needed for crystallization. The biological activity of the enzyme remains unchanged during the process and a high crystal yield is achieved. Tetragonal and rod-like lysozyme crystals, presumably of orthorhombic type, are crystallized at different boiling temperatures. This crystallization technique is proposed as a promising option to crystallize proteins economically, controllably, and gently. This method may replace standard salt-out steps in existing production processes.