The air pollutant levels of iron, manganese, lanthanum, and gold increased in 1979-1980 and were generally higher during autumn and winter in the Boyana Church in Sofia, Bulgaria. The average annual concentrations of sulfur dioxide were 0.313 mg/m³. Thiobacillus was found in the mural paintings for which sulfur dioxide serves as an energy substrate. Ammonia, also a bacterial energy substrate, had an average annual concentration of 0.021 mg/m³. Oxidant and dust average annual concentrations were 0.404 and 0.784 mg/m³, respectively. The biodeteno-ration of mural paintings due to the pollutants is discussed. -- AATA