Stone monuments situated in the Alexander Nevsky Abbey, the Summer Garden, and the Smolenskoe Cemetery, St. Petersburg, and marble and limestone sculptures and tombstones situated in the Novodevichy Convent and the Donskoy Monastery, Moscow, were investigated for their microbial contamination. The architectural objects studied date back to the 12th century. The monuments in the Alexander Nevsky Abbey were found to be severely contaminated with micromycetes belonging to 24 genera (primarily of the class Deuteromycetes). The analysis of the samples taken from the monuments of the Donskoy Monastery by the serial dilution technique showed that they were contaminated with bacteria at a density of (1-1.7) × 10⁵ cells/g. This value, however, turned out to be 1 to 2 orders greater when the bacterial population was evaluated by the luciferin-luciferase method. We succeeded in identifying 12 bacterial genera; however, this number may be increased in the course of further studies. Some preventive measures to control the biodeterioration of stone heritage are discussed.