The effect of the bacteriolytic preparation "Lysoamidase" on Staphylococcus aureus 299 P was studied. The maximum activity of the preparation was observed at pH 8.0 ionic strength 0.01-0.02 M and 50-60 degrees of the incubation medium. The electron microscopic examination revealed that "Lysoamidase" hydrolyzed the cell wall in one or several points with the following osmotic shock and extrusion of the cytoplasm. In an isotonic solution (1 M sucrose) "Lysoamidase" caused protoplast formation.