Surveys of the distribution of the lichens Xanthoria panetina and Ramalina duriaei in Israel showed that environments with air pollution had no damaging effectson X. panetina, whereas R. duriaei had disappeared from polluted environments: physiological studies supported this relative sensitivity. Investigations of possible defence mechanisms protecting X. parietina from the damaging effects of air pollution showed a multitude of possible protective systems. These included constitutive avoidance such as: efficient buffering capacity; a relatively high potassium content; and antioxidation by parietin, and induced tolerance such as: SO2 oxidation to non-toxic sulphate; increased glutathione content; induced praline and arginine synthesis; and increased detoxification of active oxygen forms.