AbstractPeroxidase activity (p. a.) of several tree species was investigated in a forest influenced by F‐containing exhalates of an aluminium smelter. P. a. increases with age of tissue, particularly under the influence of fluorides. It was used therefore to map the zone of plant reaction in apricot orchards in the vicinity of an aluminium smelter and it supplements foliar analysis.P. a. in foliage of white ash was used to determine whether exhaust of automotive traffic in the city of Zurich does affect vegetation. A significant increase of p. a. was found in the city although the plants did not show any symptoms of injury.P. a. thus is a very sensitive indicator of plant reaction to air pollutants and allows to detect “hidden” injury (no visible symp