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The susceptibility to irradiation-assisted stress corrosion cracking (IASCC) at relatively low dose is dominated by conventional mechanisms such as radiation-induced segregation, hardening, creep/relaxation and radiolysis. Large amount of helium (He) can be accumulated in reactor internal components made of nickel containing steels of pressurised water reactors after long-term operation. The increase of He content has been correlated to IASCC susceptibility in reactor internals subjected to neutron irradiation. However, the question whether He increases or even dominates the IASCC susceptibility at high doses is still open. In this work, He was homogeneously implanted at 300°C to 300 and 1000 appm. Slow strain rate tests were performed in as-implanted solution annealed and cold-worked samples and in post-implantation annealed samples. The results show that homogenised implanted He up to 1000 appm alone (without significant displacement damage) does not produce intergranular cracking and IASCC in high-temperature air and in hydrogenated high-temperature water, respectively. [ABSTRACT FROM PUBLISHER]