Abstract Results are given of a preliminary investigation of the microstructure of a commercial Mn 8%/Cr 19%/Ni 7% austenitic steel (ICL 016) before and after irradiation with 46 MeV nickel ions. Pre-irradiation phases observed were Cu-rich precipitates (d ~ 10 nm) and α-MnS phase. A surface-localised ferromagnetism observed after annealing or irradiation was found to be due to α'-martensite formed as a result of an increase in the γ/ga' transformation temperature due to evaporation of austenising elements such as Mn. Ion irradiation to 60 dpa at 625°C resulted in void-swelling of ~ 7% in solution-treated alloy containing 10 appm He. whereas swelling of ~ 1.8% occurred in the absence of helium. Irradiation also resulted in the formation of thin lath-like precipitates and the coarsening of the Cu precipitates. The results indicate that this manganese-containing alloy has an average swelling response when helium is present, with an indication that swelling can be reduced by pre-ageing at 700°C. In the ST or STA condition the alloy does not seem to offer any advantage in terms of void-swelling over other Fe-Cr-Ni austenitic steels currently favoured for LMFBR applications. The swelling sensitivity of the alloy to helium and the tendency to induced surface ferromagnetism indicate the need for further study before selecting this type of alloy for use in fusion reactors.