The antiviral activity of a wide range of naturally occurring flavonoids was investigated in vitro. Chrysosplenol B and chrysosplenol C, which are contained specifically in Chrysosplenium plants, and axillarin showed potent antiviral activity, especially against rhinovirus. A comparison of the activities of the compounds tested indicated that 3-methoxyl and 5-hydroxyl groups in the flavone skeleton were both necessary for antiviral activity against rhinovirus, and the activity may also be affected by various groups at other positions. The other flavonoids tested had little or no antiviral activity against herpes simplex virus, influenza virus and rhinovirus. These results suggest that Chrysosplenium plants, which contain large amounts of chrysosplenol B and chrysosplenol C, may be useful as medicinal herbs against the common cold caused by rhinovirus infection. These plants have not so far been used as a medicinal herb or as a folk medicine, as far as is known.