The antimutagenic activity of a methanol extract of Cassia tora seeds against aflatoxin B1(AFB1) was demonstrated with the Salmonella typhimurium assay. The numbers of revertants per plate decreased significantly when this extract was added to the assay system using Salmonella typhimurium TA100 and/or TA98. The MeOH extract was then sequentially partitioned with CH2Cl2, n-BuOH and H2O. The CH2Cl2 and n-BuOH fractions possessed antimutagenic activity but the H2O fraction was inactive. Neither the MeOH extract nor its fractions were capable of inhibiting the direct-acting mutagen N-methyl-N'-nitro-N-nitrosoguanidine suggesting that these fractions may prevent the metabolic activation of AFB1 or scavenge the electrophilic intermediate capable of inducing mutations. Column chromatography using silica gel yielded pure chrysophanol, chryso-obtusin, and aurantio-obtusin from the CH2Cl2 fraction and cassiaside and rubro-fusarin gentiobioside from the n-BuOH fraction. Each of these compounds demonstrated significant antimutagenic activity.