Microorganisms contribute substantially to the degrdn. of ceramic materials such as natural stone, concrete, ceramics and glass. Active attack is caused by excreted mineral or org. acids. Passive biodeterioration results from excreted hydrophilic slimes of heteropolysaccharides and/or protein (biofilm). All microorganisms may be of importance in biodeterioration-chemolithotrophic and chemoorganotrophic bacteria, cyanobacteria, algae, fungi and lichens. Recently it has become possible to test the resistance of ceramic materials to microbial attack. Using a simulation app., it is possible to demonstrate the interactions between a microorganism and its ceramic substratum. Purely chem. and/or phys. testing of materials is not sufficient to det. their resistance to a biol. induced attack. Biogenic sulfuric acid corrosion and nitric acid corrosion simulation expts. have demonstrated differences in the resistance of various concretes, which chem. testing failed to reproduce. Thus, biotest systems allow selection from many different materials so as to find the most appropriate.