Boiled-water extracts from 101 Korean medicinal plants were tested in vitro for their inhibitory activity against influenza virus type A by means of a modified hemagglutination inhibition test. Thirteen of the 101 extracts exhibited strong anti-influenza virus type A activity at concentrations of less than $780;{mu}g/ml$. Out of the above 13 extracts, MW-40 (Chaenomeles speciosa), MW-88 (Citrus junos), and MW-100 (Zingiber officinale) exhibited marked antiviral activity in the concentration range of $0.195;{mu}g/ml$ to 100 mg/ml, $0.0487;{mu}g/ml$ to 100 mg/ml, and $0.0487;{mu}g/ml$ to 100 mg/ml, respectively. The extracts MW-88 and MW-100 were not cytotoxic to red blood cells, whereas MW-40 showed very weak cytotoxicity in the concentration range of 50 mg/ml to 100 mg/ml. Therefore, the present results demonstrate that boiled water extracts of 2 Korean medicinal plants, MW-88 and MW-100, have strong anti-influenza virus type A activity and no cytotoxic effects, and they may inhibit attachment of the virus to the cell and may be used for prophylaxis.