Lycorine (1), one of the main alkaloids of the Amaryllidaceae family, was found to be responsible for the pronounced antiviral activity of the crude extracts from the roots and leaves of Clivia miniata Regel. It was shown that the inhibitory activity of lycorine on the cytopathogenic effect caused by a DNA and several RNA viruses on VERO cells was not virucidal. Poliomyelitis virus inhibition occurred at lycorine concentrations as low as 1 microgram/ml, whereas concentrations exceeding 25 micrograms/ml were found to be cytotoxic. Clivimine (2), clivonine (3) and cliviamartine (4) were also isolated and characterized, but these alkaloids exhibited no antiviral properties.