Lysozyme (EC 3.2.1.17, muramidase, N-acetylmuramide glycanhydrolase; systematic name peptidoglycan N-acetylmuramoylhydrolase) is an antimicrobial enzyme produced by animals that forms part of the innate immune system. It is a glycoside hydrolase that catalyzes the following process:

Hydrolysis of (1→4)-β-linkages between N-acetylmuramic acid and N-acetyl-D-glucosamine residues in a peptidoglycan and between N-acetyl-D-glucosamine residues in chitodextrins

Peptidoglycan is the major component of gram-positive bacterial cell wall. This hydrolysis in turn compromises the integrity of bacterial cell walls causing lysis of the bacteria.

Lysozyme is abundant in secretions including tears, saliva, human milk, and mucus. It is also present in cytoplasmic granules of the macrophages and the polymorphonuclear neutrophils (PMNs). Large amounts of lysozyme can be found in egg white. C-type lysozymes are closely related to α-lactalbumin in sequence and structure, making them part of the same glycoside hydrolase family 22. In humans, the C-type lysozyme enzyme is encoded by the LYZ gene.

Hen egg white lysozyme is thermally stable, with a melting point reaching up to 72 °C at pH 5.0. However, lysozyme in human milk loses activity very quickly at that temperature. Hen egg white lysozyme maintains its activity in a large range of pH (6–9). Its isoelectric point is 11.35. The isoelectric point of human milk lysozyme is 10.5–11.