The earliest therapeutic use of artificially produced radioactive isotopes in medicine was in the field of hematology, and dates back to the Summer of 1936 when radiophosphorus was administered to a patient with chronic lymphatic leukemia. This was twenty years ago when radioisotopes first became available for medical investigations from the 37-inch cyclotron in Berkeley (11). Since that time, and especially in recent years with the development of the nuclear reactor and its immense capacity for producing radioactivity, there has been an increasingly widespread use of radioisotopes throughout the world in the study, diagnosis, and treatment of hematological and other diseases.