A CONSIDERATION of the energetics of radioactive decay shows that two isotopes of the same mass number but neighbouring atomic number cannot both be stable if the rest mass of the neutrino is zero. A number of such neighbouring isobaric pairs exists in Nature; in some cases the expected radioactivity has been found in one member of the pair, with a period of decay comparable to, or longer than, the age of the earth (for example, the well-known case of 4019K). The two naturally existing isotopes of indium—11349In and 11549In (abundances 4.5 and 94.5 per cent respectively)—are each members of such isobaric pairs, the first with 11348Cd, and the second with 11550Sn. Accordingly, it was decided to search for radioactivity among these elements, as none had hitherto been found.