The extraction behavior of lysozyme from chicken egg white by PEG/salt aqueous two-phase system (ATPS) was investigated. Considering the excluded volume effect and the hydrophobic character, polyethylene glycol (PEG) with an intermediate molecular weight (6000) was used to form the ATPS. Sulfate was chosen as the phase-forming salt because of its ability of promoting the hydrophobic difference between the phases. Optimal conditions of the process were investigated using the Box-Wilson experimental method. It was found that the ATPS should be composed of 16.1% (w/w) PEG and 12% (w/w) sodium sulfate, with addition of 500 mM of sodium perchlorate. And the egg white should be diluted 7.6-fold before being extracted by the ATPS. When this system was operated at room temperature (25 degrees C) and pH 10, approximately 70% of lysozyme could be extracted from the diluted chicken egg white. The specific activity of lysozyme resulting from the partitioning was estimated to be 39 500 unit/mg protein. (c) 2005 Elsevier Ltd. All rights reserved.