Abstract: It is well known that many components of herbal products are mutagens. Because of increasing use of medical herbs, in the present study the genotoxicity of Gasterolan was investigated using cultured human lymphocytes and rat bone marrow. Gasterolan (Goldaru Company, Esfahan, Iran) is used as an antispasmodic, carminative and for spastic disorders of the gastrointestinal tract. The drug contains hydroalcoholic extracts of Foeniculum vulgare fruits, Matricaria chamomilla flowers and Mentha piperita. The cultured human lymphocytes were treated with 0.5 and 0.75% (v/v) of Gasterolan and 0.75% of ethanol for 14 h. Chromatid break was used as a marker for damage. Ethanol did not increase the frequency of chromatid break (Z = 0.0068, p = 0.948). The frequency of chromatid break is significantly increased in 0.5 (Z = 2.75, p = 0.006) and 0.75 (Z = 7.47, p<0.001) percent of Gasterolan. Considering that ethanol does not increase the frequency of chromatid break, the observed effect is probably due to some components present in the herbal product. Investigation of rat bone marrow metaphases show that chromatid breaks increased as a function of harvest time (from 2-8 days after beginning the treatment) (r = 0.998, df = 2, p = 0.002). More studies are necessary to find components that increased chromosomal aberrations.