this invention relates to a compound or group of compounds present in an active principle derived from plants of species euphorbia peplus , euphorbia hirta and euphorbia drummondii , and to pharmaceutical compositions comprising these compounds . extracts from these plants have been found to show selective cytotoxicity against several different cancer cell lines . the compounds are useful in effective treatment of cancers , particularly malignant melanomas and squamous cell carcinomas . in a preferred embodiment of the invention , the compound is selected from the group consisting of jatrophanes , pepluanes , paralianes and ingenanes , and pharmaceutically - acceptable salts or esters thereof , and more particularly jatrophanes of conformation ii .