Lysozyme (LZ, muramidase, N-acetylmuramylhydrolase) is a protein occuring in animals, plants, bacteria and viruses. It can be found e.g. in granules of neutrophils, macrophages and in serum, saliva, milk, honey and hen egg white. The enzyme hydrolyzes the β-1,4 glycosidic bonds between N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) of cell wall peptidoglycan (PG) in Gram-positive and Gram-negative bacteria. In the animal kingdom, three muramidase types have been identified: the c-type (chicken type), the g-type (goose-type) and the i-type (invertebrates). The c-type LZ from hen egg white is a model for the study of protein structure and function. Muramidase shows bactericidal activity mainly against Gram-positive bacteria. Cytolytic activity against cells of Gram-negative bacteria has not been proved. Bacterial cells have developed defense mechanisms that allow them to avoid the action of LZ. They are based e.g. on the production of enzyme inhibitors or modification of the PG. LZ is one of the most studied enzymes and yet not all aspects characterizing this protein are fully understood. One of the most important unresolved issues concerning the biological function of LZ is the role of muramidase in the bactericidal action of serum against Gram-negative bacteria. In order to clarify the function of LZ, the enzyme is e.g. removed from the serum by adsorption onto bentonite (montmorillonite, MMT). By using X-ray diffraction techniques it has been shown that MMT after contact with the serum is delaminated. The problems associated with folding of muramidase and LZ participation in the development of amyloidoses also await explanation.