The effect of industrial pollution upon stone has been thought to be principally chemical in origin. Much damage, however, is biological. Bacteria, yeasts, microfungi, and even algae are converting hard marble (calcium carbonate) into soft plaster (calcium sulfate) and causing other deterioration by both chemical and physical means. Sulfuric, nitric, nitrous, formic, acetic, and oxalic acids are formed by the metabolism of such microbes as <i>Thiobacillus Thioparus, Nitrobacter and <i>Nitrosomonas, from atmospheric sulfur dioxide, carbon dioxide and ammonia. Mycelia from microfungi spores can also powder stone from pressure as they root, breaking stone apart. Most of this damage from biological agents is very serious and very recent, having occurred primarily within the past 30 years. The use of antibiotics and various biocides is discussed, the most promising being isothiazolinone chloride. -- AATA