Mutagenicity screening was carried out on 31 samples of popular Thai spices derived from 12 different families of plants, namely the Amaryllidaceae (2), Graminae (1), Labiatae (4), Lauraceae (1), Magnoliaceae (1), Myristicaceae (2), Myrtaceae (2), Piperaceae (3), Rutaceae (2), Solanaceae (2), Umbelliferae (2) and Zingiberaceae (9). Two variations of the rapid streak method of rec-assay in Bacillus subtilis strains H17 (rec+) and M45 (rec-) were used. Only Ceylon cinnamon (the bark of Cinnamomum zeylanicum Nees of the family Lauraceae) showed mutagenic activity. The crude form of this spice and its water-heated and water-macerated residues all produced the rec effect, while water-heated and water-macerated filtrates did not, even in concentrations equivalent to as much as 50 mg solids/test disc.