Small-sheet tensile samples of Types 316 and 405 stainless steels were encapsulated together and irradiated in EBR-II at about 400 C (752 F) to a total fluence of 3.3 × 10²² n/cm² with 2.3 × 10²² n/cm² fast (E>0.1 MeV). A variety of microstructures was produced in each alloy prior to irradiation. In Type 316 stainless steel, swelling increased with the amount of carbide precipitation prior to irradiation, implying that carbon in solution impedes void formation. Cold-working 25 percent suppressed void formation. No swelling was observed in Type 405 stainless steel. Irradiation caused an increase in the yield and tensile strengths, and a decrease in ductility in all cases except the cold-worked Type 316 stainless steel. These latter samples showed a decrease in yield and tensile strengths and an increase in ductility due to the reversion of ϵ martensite to austenite.