The use of the new radioactive isotopes in clinical medicine has introduced some new hazards and intensified some old ones. Many hospital organisations used to handling large quantities of radium and X rays are familiar with the risks of external irradiation, but are not familiar with those arising from the use of watery solutions or solids not enclosed in sealed containers. It is necessary therefore to extend organisation, to introduce new precautions, and to educate staff in the new outlook. The clinical problems may be thought to be simpler than those arising in Atomic Energy Projects inasmuch as the very difficult questions of α-ray emitters and particularly α-ray dusts do not arise, but on the other hand the quantities of β- and γ-ray emitters now used clinically are frequently involve hundreds of millicuries of high energy γ-ray emitters and the new telecurie sources may well reach thousands of curies.It is wrong, therefore, for those who are embarking on therapeutic investigations to assume that the elaborate precautions known to be taken in Atomic Energy Projects are not necessary in clinical practice.