In the following article a British physicist speculates on the composition and possible radiological effects of the superbomb tested in the Pacific last spring. He concludes that it “is a kind of cobalt bomb; in some respects, it is even worse.” Mr. Rotblat, executive vice-president of the British Atomic Scientists' Association, is professor in the Physics Department of the Medical College, St. Bartholomew's Hospital, London. His article is reprinted from the March issue of the Atomic Scientists Journal.