Bacteria, fungi, algae, and actinomycetes were quantitatively determined in the uppermost layers of sandstones of several monuments within Germany before and after cleaning the stone surface by using high pressure water blasting. Dilution and inoculation for counts was performed using a spiral plater technique. The results of the study indicate that, depending on the original distribution of microorganisms, the microflora of sandstones immediately after treatment is not drastically altered, especially with regard to specific groups of microorganisms, which were isolated and counted under oligotrophic conditions. After a short period of time, microorganisms again reach their previous number or even exceed it. Cleaning by the conventional methods cannot be regarded as a procedure which reduces microbial colonization indefinitely.