The antimutagenic activities of the oils obtained from leaves and fruits of Fagus orientalis have been shown in experiments with spontaneous and mutagen- and ageing-induced variability. The aberrations of chromosomes in the meristematic cells of the Allium cepa L., Vicia faba L., Triticum aestivum L., and marrow cells of Vistar rats as well as Arabidopsis thaliana gene mutations have been mobilized as experimental tests.