The application of five water-soluble, halogen-free, alkylammonium-based ionic liquids (ILs) as additives for advanced crystallization of lysozyme was investigated. Their biocompatibility was determined by long-term measurement of the overall mean relative enzyme activities. These were maximally reduced by about 10-15% when up to 200 g IL l(-1) was added. Sitting-drop vapor diffusion crystallization experiments revealed that the addition of some of the ILs led to less crystal polymorphism and precipitation was avoided reliably even at larger NaCl concentrations. The addition of ILs tended to result in larger crystals. The kinetics of lysozyme crystallization were significantly enhanced using ILs as crystallization additives, e.g. by a factor of 5.5 when 100 g ethanolammonium formate l(-1) was added. ILs with "soft" anions, such as formate or glycolate, were superior to ILs with "hard" anions, like nitrate.