ABSTRACTA control experiment showed that cottonseed oil used in frying falafel developed significant mutagenic activity. As a result, one hundred samples of fresh and used cottonseed oil employed commercially for frying falafel were collected from restaurants in thirteen major cities of Egypt. Samples were examined for mutagenic activity using the Salmonella typhimurium assay with and without S9. Six of the heated oil samples showed significant mutagenic activity. None of the fresh unheated oil samples were mutagenic. Salmonella typhimurium TA 102 gave a higher response than did either TA 100 or TA 98.Peroxide, hydroxyl, acid and conjugated diene values were much higher for mutagenic samples. Iodine values, however, were lower. Liquid chromatography fractionation revealed that the mutagenicity was concentrated in the polar fraction. The column fraction having the highest mutagenic activity was further examined by chromatographic and spectroscopic techniques.