Describes some of the main findings of a systematic study of the effect of grain size on the void volume swelling. In this study a powder-produced 20 Ni/20 Cr austenitic stainless steel, with 0.02% carbon and without carbide-forming elements was used. Some specimens containing dispersions of alumina particles (~0.8×1015 particles of 500 Å diameter cm-3) were also irradiated. For theoretical and practical convenience, high energy electron irradiation was used to study the formation and growth of voids in these materials. All the irradiation experiments were done at 600°C in the EM-7 1 MeV electron microscope at Harwell. Before irradiation 10 p.p.m. of helium was injected into the samples using a 100 kV heavy ion accelerator.