Quercetin, a flavonoid, is found in many fruits and vegetables. This drug was previously shown to affect the metastatic potential of mouse tumor cells. Mutagenicity of quercetin was examined by means of DNA fingerprint analysis using the Pc-1 probe that efficiently detects mutations due to recombination. Treatment of BMT-11 and FM3A tumor cells with 55 microM quercetin resulted in gain and loss of bands in the fingerprints in both cell lines. The frequencies of the clones having undergone mutation were 3/11 and 6/26, respectively. This suggests that quercetin is mutagenic and induces recombination. This result seems to provide a molecular basis for the phenotypic variations of BMT-11 tumor cells induced by quercetin.