The efficacy of Koretrel [copyright], a biocide used in south-eastern Asia to remove epilithic photoautotrophic organisms, was tested in the field and laboratory using lichens as target organisms. In situ observations were carried out at increasing time intervals for 6 month on 15 vegetation plots and 21 saxicolous specimens representative of the lichen flora of Italian monuments, selected according to their different ecology, growth-form and distribution. Change in chlorophyll fluorescence was monitored in three species for 32 day after biocide treatment. The effects of the biocide on the substratum was studied by monitoring changes in capillary water absorption and surface colorimetry of two CaCO3-containing rocks (Aurisina limestone and Muggia sandstone), and a grey granite. Rock crystal integrity was monitored by SEM. The results show that Koretrel [copyright] is very efficient in killing the lichens. Pronounced effects were observed in all species after 1-2 weeks, with chlorophyll fluorescence reduced to zero in 4 day in the case of Lecanora muralis and Neofuscelia loxodes and in 8 days for Caloplaca saxicola. The biocide considerably decreases capillary water absorption, and alters the surface colour of the substrata, particularly for Aurisini limestone. However, no corrosion features were observed in the rock minerals.