AbstractThe detection of hidden F‐injury of forest trees by a simple colorimetric determination of peroxidase actívity. The paper describes a simple routine method for determining colorimetrically peroxidase activity (p. a.) in foliage of forest tree species, including conifers. The effect of factors such as foliage age, F Content, external F dust, necrosis etc. on p. a. is investigated. Analyses of plants exposed at different distances to F exhalates of an aluminum plant show that with decreasing distance F content and p. a. increase. P. a. thus is an indicator of air pollution effects on tree physiology even in the range where no visible symptoms of injury occ