Extracts from Ulex europaeus L. (Fabaceae) were tested for antiviral activity against HSV-1 and poliovirus. It was found that the n-butanolic extract significantly inhibited the replication of these viruses, as shown by inhibition of cytophathic effects and reduction of plaques; the extract has neither a prophylactic nor a virucidal activity. From this extract has been isolated and identified by chromatographic, chemical, and spectroscopic methods, the 5-O-methylgenistein 7-O-beta D-glucopyranoside. This isoflavonic glycoside has not been previously described and it is responsible at least partially for the antiviral activity.