A new method for the evaluation of microbial activity on the stone of buildings and historic monuments is described. The modified assay of dehydrogenase activity (DHA) is shown to be useful in the study of the influence of microorganisms on the decay of stone. The method complements the usual microbiological examination procedures, providing a rapid quantification of the metabolic activity of microbial communities in the surface layers of the stone. The enzyme test can also be used to assess the influence of different nutrient compounds on the metabolism of microorganisms. Thus the effect of the accumulation of biogenic residues of air pollution, or of the application of treatments to protect the stone, on the biodeterioration activity of the microorganisms colonizing the stone can be elucidated. Simultaneously the spatial distribution of the microorganisms over the rock profile can be made visible by the use of a red indicator for the DHA. The method provides an excellent and easy-to-handle tool for the study of changes in the microflora, which are caused by all kinds of conservation treatment and which may lead to degradation of the host stone.