Abstract: The invention relates to a method for fighting mold, algae, and other microorganisms on various wall, floor, or ceiling surfaces infested with said organisms, and on masonry in general. The method comprises the following steps carried out consecutively: applying a first solution to the infested surface, wherein the first solution contains at least one surfactant compound; applying at least once a second solution that was basified by means of caustic soda to the infested surface, wherein the solution contains at least one hypochlorite compound; applying a third solution to the infested surface, wherein the solution contains at least hydrogen peroxide and lactic acid as active substances, and wherein the application of the third solution is continued until the foam formation on the infested surface has ended; applying a fourth solution to the infested surface, wherein the solution contains at least one quaternary ammonium compound.