Lycorine, one of the main alkaloids of the Amaryllidaceae, was found to be responsible for the pronounced antiviral activity of crude extracts from the roots and leaves of C. miniata Regel. The inhibitory activity of lycorine on the cytopathic effect caused by herpesvirus, poliovirus, coxsackievirus, measles virus and Semliki Forest virus in African green monkey kindey Vero cells was not virucidal. Poliovirus inhibition occurred at lycorine concentrations as low as 1 .mu.g/ml; concentrations > 25 .mu.g/ml were cytotoxic. Clivimine, clivonine and cliviamartine were also isolated and characterized, but these alkaloids exhibited no antiviral properties.