The enzyme system composed of human neutrophilic myeloperoxidase (H2O2-oxidoreductase, EC 1.11.1.7), H2O2 and Cl-, at pH 4.5 interacts with egg white lysozyme (EC 3.2.1.17) in several stages. In the first stage, occurring at lysozyme to H2O2 molar ratio of 1:1.4-1.8, the lysozyme loses its enzyme activity but does not yield any derivative distinguishable from the native protein on polyacrylamide gel electrophoresis (PAGE). The second stage of oxidation begins at lysozyme to H2O2 molar ratio above 1:5, producing a change in the lysozyme spectrum at 260-290 nm, and yielding protein derivatives with molecular masses equal to multiples of 14.3 kDa, i.e. the lysozyme molecular mass. This implies that an excessive oxidation of lysozyme by the myeloperoxidase-H2O2-Cl- system produces cross-linking of lysozyme molecules to di-, tri-, tetra-, and pentameric structures. At lysozyme to H2O2 molar ratio exceeding 1:12 a water insoluble white product, which consists of a set of lysozyme cross-linked derivatives, is obtained.