Thin foils of an ‘ experimental ’ austenitic stainless steel, with and without dispersions of aluminium oxide particles, are irradiated with 1 MeV electrons in a High Voltage Electron Microscope at 600°C. Evidence of grain size dependent void nucleation, void concentration, and void volume swelling are presented for grains in the size range 0°45 to 50 μ. In both undoped and helium-doped samples the void nucleation is delayed, void concentration is lowered, and void volume swelling is reduced by decreasing the size of the grain under irradiation. The results are discussed in terms of a ‘ defect depletion ’ model based on the property of grain boundaries as neutral and unsaturable sinks for vacancies and self-interstitials. It is suggested that even in the presence of sufficiently large amount of impurity gas atoms, a critical level of vacancy supersaturation is necessary to produce critically sized vacancy clusters which upon stabilization become viable void nuclei. It is concluded that the presence of stable and small grains improves the damage resistance of the material tremendously.