The effects of irradiation on high-temperature mechanical properties of structural materials is described using Type 304 stainless steel as an example. The general effect is one in which the grain-boundary fracture process, but not the deformation process, is affected. The data suggest the primary cause to be helium generated from (n,α) reactions. Several metallurgical techniques for improving the ductilities of irradiated alloys are suggested, and experimental data on Type 304 stainless steel are given for which the degree of improvement is demonstrated.