Antigenotoxic properties and the possible mechanisms of water extracts from Cassia tora L. (WECT) treated with different degrees of roasting (unroasted and roasted at 150 and 250 degrees C) were evaluated by the Ames Salmonella/microsome test and the Comet assay. Results indicated that WECT, especially unroasted C. tora (WEUCT), markedly suppressed the mutagenicity of 2-amino-6-methyldipyrido(1,2-a:3':2'-d)imidazole (Glu-P-1) and 3-amino-1,4-dimethyl-5H-pyrido(4,3-b)indole (Trp-P-1). In the Comet assay performed on human lymphocytes, WECT exhibited significant protective effect on Trp-P-1-mediated DNA damage followed the order of unroasted (55%) > roasted at 150 degrees C (42% ) > roasted at 250 degrees C (29%). Pre-treatment of the lymphocytes with WEUCT resulted in 30% repression of DNA damage. However, no significant effect on excision-repair system was found during DNA damage expression time in post-treatment scheme (p>0.05). WEUCT showed 84% scavenging effect on oxygen free radicals generated in the activation process of mutagen detected by electron paramagentic resonance system. Two possible mechanisms were considered: (1) neutralization the reactive intermediate of Trp-P-1; and (2) protecting cells directly as an antioxidant that scavenge the oxygen radicals from the activation process of mutagen. The individual anthraquinone content in extracts of C. tora was measured by HPLC. Three anthraquinones, chrysophanol, emodin and rhein, have been detected under experimental conditions. The anthraquinone content decreased with increased roasting temperature. Each of these anthraquinones demonstrated significant antigenotoxicity against Trp-P-1 in the Comet assay. In conclusion, our data suggest that the decrease in antigenotoxic potency of roasted C. tora was related to the reduction in their anthraquinones.