The lichen Caloplaca aurantia, when growing on concrete roof tiles, accumulates high concentrations of heavy metals. The uptake correlates with the ambient metal concentration. The results of lichen and substratum analyses of samples from a heavily polluted area indicate that the concentrations of metals in the lichen are many times higher than in the substratum. The relative content of the eight metals, Fe, Mn, Zn, Cu, Pb, Cr, Ni and Cd was determined. The content of Fe, Cr, Ni and Cd was not significantly different in the various tile zones sampled. Fe, Mn, Zn, Cu, Cr, Ni and Cd were mainly taken up by the lichen directly from the atmosphere. The uptake of these metals from the tile is almost negligible. The maximum uptake of Pb by C. aurantia from the tile glaze was 10.95°° of the total amount in the lichen. The presence of the lichen on the tiles prevents the penetration of Pb, Zn and Mn into the substratum.