The tensile and creep-rupture properties of Types 304 and 304 + 0.15% Ti stainless steels have been determined after irradiation at temperatures in the range 400 to 820°C to maximum neutron fluences of 6.1 × 1022 n/cm2 (>0.1 MeV). Changes in mechanical properties were related to the microscopic observations of irradiation-produced defects. When irradiated in the annealed condition in the neighborhood of 450°C, Type 304 stainless steel exhibited an increased yield stress, reduced strain hardening coefficient, and reduced uniform and total elongation. The increased yield stress could be correlated with the strengthening expected from irradiation-produced voids and dislocations. With increasing irradiation temperature the concentration of these defects decreased and thus the magnitude of the yield stress increase became less. At 500 to ∼600°C irradiation and test temperatures a pronounced reduction in creep-rupture ductility was observed. Fractures were inter granular. It is suggested that in this temperature range the void-dislocation structure together with the transmutation-produced helium were responsible for the intergranular fractures and low ductilities. At higher temperatures no void-dislocation structures were formed. For these conditions the ductilities were higher than in the 500 to 600°C range but still significantly below the unirradiated value. Variation in alloy composition and pre-irradiation micro structure had a strong influence on the postirradiation properties. Type 304 + 0.15% Ti stainless steel exhibited significantly higher tensile and creep-rupture ductilities than the standard alloy when irradiated and tested above 450°C. The strength properties of specimens irradiated in the 10% cold-worked condition were similar to those of material irradiated in the annealed condition, but the total elongation and reduction in area were slightly lower.