A review of the approaches to experimental work on the role of bacteria in stone decay with a discussion of the results of bacteriological surveys on sandstone from Portchester Castle and Tintern Abbey (England). Some of the principal decay species of bacteria could be detected in large unmbers on severely decayed stone, and it was found that the main cause of damage was acid production by the bacteria which attacks the calcium carbonate matrix of the stone. The authors discuss the validity of Koch's postulates in the study of bacterial effects on stone. Diagrams. Micrographs. Data table. Bibliography (31 entries).