Tension and stress-rupture specimens of Hastelloy X-280, a solution-hardenable nickel-base alloy, were given various preirradiation experimental thermomechanical treatments. The specimens were subsequently irradiated at 540 F to exposures of approximately 1 × 10²⁰ nvt (E > Mev) and 3 × 10²⁰ nvt thermal. Tension tests were performed at room temperature and 1350 F in air and the stress-rupture tests were performed at 1350 F in air. The tension tests showed that, when compared with a standard commercial treatment, the experimental treatments generally showed reduced radiation damage. Some treatments increased postirradiation high temperature tensile ductility by 100 per cent while maintaining strength. One treatment resulted in increases in postirradiation high temperature yield strength and ultimate strength of 100 per cent and 25 per cent, respectively, with only a 2 to 4 per cent ductility loss. Further, a maximum increase of 57 per cent in postirradiation room temperature yield strength was possible while maintaining at least 18 per cent uniform elongation. The stress-rupture tests showed that some experimental treatments increased postirradiation rupture life up to 106 per cent over that of the irradiated standard treated material with no change in postirradiation minimum creep rate. These experimental treatments increased duration of postirradiation secondary creep up to 107 per cent and increased the duration of postirradiation tertiary creep up to 115 per cent. Metallography of broken tension specimens showed evidence of strain-induced precipitation, irradiation-induced precipitation, and combined strain- and irradiation-induced precipitation. Property improvements are interpreted on the basis of the interaction of the microstructures formed by the preirradiation treatments with the neutron environment and subsequent thermal and stress environments associated with the testing. Relationships of these results to reactor material fabrication are considered.