Fast-neutran irradiations in the EBR-II have been completed an biaxial stress rupture, creep, and tensile specimens of AISI 304 and 316 stainless steel. Postirradiation test results show that irradiations in the 480 to 650°C range to fluences of 1×1022n/cm2(E>0.1 MeV) substantially reduce the time-dependent rupture life and ductility of these materials. Tensile ductility is also severely reduced.Bulk-density measurements and electron-microscopy examinations on specimens of annealed 304 from EBR-II core components and mechanical property specimens have been made for fluence levels to 7×1022n/cm2and at temperatures in the 360 to 470°C range. Both the bulk-density measurements and microscopy examinations correlate well and indicate that volume changes of 4% can be expected under these conditions. The temperature and fluence dependency for annealed 304 stainless steel has been determined and can be expressed as:The mechanisms responsible for the observed degradation of mechanical properties and metal swelling are being studied. Some observatians are presented. However, as yet, no adequate nucleatian and growth model has been determined to enable an acceptable extrapolatian of these data-to-goal fluence levels to be achieved in Liquid Metal Fast Breeder Reactor core companents or fuel-pin cladding.