[en] The International Atomic Energy Agency published in 1958 a Manual entitled ''Safe Handling of Radioisotopes'' (Safety Series No. 1 - STI/PUB/1), based on the work of an international panel convened by the Agency. As recommended by that panel and approved by the Agency's Board of Governors, this Addendum has now been prepared, primarily as a supplement to the Manual. It contains information necessary to medical officers concerned with the implementation of the controls given in the Manual. In addition, it is intended to serve as a brief introduction to the medical problems encountered in radiological protection work and to the methods of resolving them. As in the case of the Manual itself, the information given in this Addendum is particularly relevant to the problems encountered by the small user of radioisotopes. Although the basic principles set forth in it apply to all work with radiation sources, the Addendum is not intended to serve as a radiological protection manual for use in reactor installations or large-scale nuclear industry, where more specialized techniques and information are required.