Abstract : Helium was injected into small tensile samples of Type 316 stainless steel by alpha-particle irradiation from a cyclotron. Subsequent tensile testing in the temperature range 540 to 815 deg C revealed a loss of ductility, as measured by elongation at failure. The lowest temperature at which a ductility loss was manifest depended upon the microstructure of the samples. A fine dispersion of sigma particles within the matrix was capable of raising this temperature by retarding the accumulation of helium at grain boundaries. Reduced ductility was always accompanied by partial or complete intergranular failure and the presence of intergranular cracks. These cracks originated as small voids adjacent to grain-boundary carbide particles, presumably through the action of grain-boundary sliding. Large helium bubbles found attached to carbide particles are responsible for the relative ease of void formation, as compared to samples without helium.