An extract of Ribes nigrum L., known as blackcurrant in Europe and Kurokarin(R) in Japan, has been used as an ingredient in a variety of foods and folk medicine. In this study, the anti-herpesvirus activity of this extract was examined in vitro. The extract inhibited herpes simplex virus type I attachment on the cell membrane completely at a 100-fold dilution, as well as the plaque formation of herpes simplex virus types I and 2, and varicella-zoster virus by 50% at a 400-fold dilution or lower concentrations. This latter activity, which inhibits virus replication in cells, was due to the inhibition of protein synthesis in infected cells from the early stage of infection. Kurokarin is a possible candidate as a herbal medicine for herpesvirus infectious diseases. Copyright (C) 2003 John Wiley Sons, Ltd.