The invention provides a method of treating tumors by adminstering to a human or animal patient an effective amount of a molecule of 2 to 4 amino acids or equivalent subunits in length, which incorporates a bulky and lipophilic group comprising at least 13 non-hydrogen atoms and containing no more than 2 polar functional groups, in which the bulky and lipophilic group incorporates one or more closed rings of 5 or more non-hydrogen atoms, and in which the molecule further has at least two more cationic than anionic moieties.