This study evaluates the genotoxic and antigenotoxic effects of the essential oil of Origanum onites L. and carvacrol that are used in medicine, flavouring of food and crop protection by Ames Salmonella/microsomal test. The mutagenic activity was initially screened using Salmonella typhimurium strains TA98 and TA100, with or without S9 metabolic activation. No mutagenicity was found in the oil to the both strains either with or without S9 mixture whereas significant mutagenic activity was induced by carvacrol generally in the absent of metabolic activity. The oil and its major constituent carvacrol were finally tested for their antimutagenic activity with 30 min standard preincubation time. It was shown that both of them strongly inhibited mutagenicity induced by 4nitro-o-phenylenediamine and 2-aminofluorene in both strains with or without S9, respectively. These results indicate significant antimutagenicity of the essential oil and carvacrol in vitro, suggesting its pharmacological importance for the prevention of cancer. (c) 2005 Elsevier Ltd. All rights reserved.