Most of the stone monuments in Hungary were built from low-quality porous limestone. Their speedy deterioration as a result of growing pollution causes great concern among stone restorers. The author discusses the main causes of crust formation and change in color and texture distorting many of the ornamental details. The importance of a careful examination of the physical and chemical properties of the stone to be cleaned is emphasized. Finally the article states the basic requirements to be met by a proper cleaning agent and describes the mostwidely used methods, giving preference to cleaning by high-pressure cold and warm water or steam. See also 21-950. -- AATA