Describes a new laboratory method for assessing the resistance of stone to algal disfigurement and the efficacy of chemical inhibitor in preventing growth of algae. The technique has demonstrated the relative susceptibilities of different types of stone to algal growth and the variable performance of a quaternary ammonium biocide on stone as well as mortar substrates. Using asbestos cement as an alternative lest substrate, the method has demonstrated the superior performance of inhibitors containing organo-tin compounds and of treatments which incorporate a herbicide into cleaning procedures.