Lysozyme is a hydrolytic enzyme having the capacity to decompose bacterial cell walls. It constitutes a mechanism of natural, primary protection for the embryo developing in the egg. The egg white is a rich source of lysozyme used to produce the enzyme on an industrial scale. The objective of this study was to determine the changes in the lysozyme of a native egg white resulting from storing either the raw whole eggs or the separated egg whites under the conditions similar to those of the egg incubation during hatching i.e. approximately at 40 °C for 20 days. It was proved that when chicken eggs were kept under the above conditions, qualitative and quantitative changes in the lysozyme present in the egg white occurred. The incubation of enzyme caused its partial oligomerisation, which resulted in the formation of a dimeric form. This phenomenon was accompanied by a decrease in the hydrolytic activity of the enzyme.