Algal and cyanobacterial growth occurring on the plaster-covered interior walls of the church of St. Stephanus at Pilsum (northern Germany) has been studied. A Mobile Ultra Violet Unit (MUVU) has been successfully applied to the church walls in order to control growth of photo-synthetic organisms. The killing effect of the UV-light on the flora has been demonstrated by the following methods: 1) infrared photography, 2) quantitative chlorophyll determinations, 3) transmitted light microscopy, 4) scanning electron microscopy and 5) plate counts. It was demonstrated that the MUVU method is efficient and cheap. The construction and operation of the MUVU is easy. Assessment of the efficiency of the process can be carried out without complicated chemical or biological analyses, by using IR-photography.