The antimutagenic effect of volatile decomposition products from thermally oxidized linoleate on mutagenesis by UV irradiation was investigated in Escherichia coli B/r WP2. When added to an agar medium, these products greatly reduced the number of Trp+ revertants. The same antimutagenic effect was observed by acrolein, 2-hexenal, 2-heptenal, 2-nonenal and 2,4-decadienal; these unsaturated aldehydes were components of volatile products.