The CHCl(3) extract of Mentha cordifolia Opiz. showed antimutagenicity against tetracycline. An antimutagen was purified by solvent partitioning and repeated normal phase-vacuum liquid chromatography (NP-VLC) using a micronucleus test-guided isolation and purification. Spectral analyses showed that the isolated antimutagen is possibly 6,7-bis-(2,2-dimethoxyethene)-2,11-dimethoxy-2Z,4E,8E,10Z-dodecatetraendioic acid. It inhibited the mutagenicity of tetracycline by 68.7% at a dosage of 0.01 mg per 20 g mouse. Statistical analysis using Kruskal-Wallis one-way analysis of variance (ANOVA) by ranks showed that its variance differs from that of the solvent control group (tetracycline+dimethylsulfoxide (DMSO)) at alpha=0.001. Moreover, the isolated antimutagen did not exhibit mutagenic activity at the same dosage. Statistical analysis showed that it is not mutagenic at 0.001 level of significance because its variance differs from that of tetracycline.