An ultrasonic technique was used to determine elastic moduli of Type-304 stainless-steel samples irradiated to fluences as high as 7.5 × 1022 n/cm2 (E > 0.1 MeV). Significant decreases in the elastic moduli were found, which correlate linearly with irradiation-induced swelling. The Young’s modulus was found to decrease ∼2.4% per 1% swelling.