The effects of fast neutron irradiation on the tensile and creep-rupture ductility of austenitic stainless steels are presented. At low temperatures, below 450°C, radiation causes a reduction in the strain hardening exponent and thus the uniform strain. At high temperatures, above 750°C, reduced ductility results from helium embrittlement. At intermediate temperatures, hardening and helium effects act synergistically to produce lower ductility than would result from either effect alone.