The effects of penicillin, lithium chloride and homologous antiserum with complement on S. aureus after a single exposure to these agents and in subsequent subculturing were studied. The viability of the altered forms obtained in these experiments was evaluated by the number of colony-forming units per ml. The action of all above-mentioned agents resulted in the appearance of staphylococcal forms with the altered cell wall. The lesions in the submicroscopic organization of the cell wall, produced by the action of the above-mentioned agents, differed in the appearance of porosity and ruptures in the wall under the action of penicillin and antiserum, thinning and peeling-off of the wall under the action of lithium chloride. The damage of the cell wall is accompanied by the disorganized septal development and mitosis, and sometimes by the formation of elementary bodies in the cytoplasm.