1. The pile will produce the majority of the radioactive isotopes of interest for tracer investigations in medicine and biology, in most instances in far larger amounts than required (see Table I). 2. The pile is essential for the production of large quantities of γ-ray sources of reasonably long life, as substitutes for radium for use in radiotherapy (see Table II). 3. The best substitute for radium made in the pile appears to be radio-cobalt, Co60, of half-life 5·3 years. The second choice is radio-tantalum, Ta182, of half-life 97 days. 4. In general, for the preparation of isotopes, the yields with the cyclotron are very much smaller than those with the pile (see Table IV). However, the cyclotron is an essential supplement to the pile in order to produce:— (a) Alternative isotopes of a given element and the few important isotopes which cannot be made in the pile because no slow or moderately fast neutron reaction is possible or practicable. (b) Higher concentrations of activity in certain instances whe...