In the process of crystallizing egg white lysozyme by cooling a salt solution containing egg white lysozyme, a seed crystal is added to the solution, which solution is then maintained at a temperature of about 10.degree. C. to 28.degree. C. for a certain period of time and subsequently cooled, whereby lysozyme can be crystallized to form an agglomerate of many individual needle crystals. The solution containing crystalline lysozyme thus obtained can be filtered within an extremely short period of time, that is, within only a few minutes.