Quercetin, an ubiquitously occurring plant mutagen was detected and quantified in a methanolic extract prepared from the herb of Alchemilla speciosa BUSER (Rosaceae). However, the extract exhibited much less mutagenicity in Salmonella typhimurium than expected from its quercetin content. A bioassay guided fractionation resulted in a crude fraction in which esculetin was present, as evidenced by GC/MS analysis. This coumarin derivative was detected for the first time in an Alchemilla species. It was at least partly responsible for the inhibition of the quercetin mutagenicity. Esculetin was also found to be active against certain mutagens which need metabolic activation. Its activity was comparable or even more pronounced than that of other plant coumarins.