The temperature dependence of the tensile lower yield stress of an annealed aluminium grain size controlled mild steel has been investigated in the range 23–250 °C and at a strain rate of 1.67 × 10−4sec−lbefore and after neutron irradiation to 2.3 × 1018n/cm2(fission). The yield stress of the irradiated steel decreases with increasing temperature due to thermal activation of the radiation damage and is predicted to reach asymptotically that of the unirradiated steel at ∼285 °C; the maximum test temperature was below that at which thermal annealing of the damage occurs. This implies that the athermal stress component due to irradiation is zero and hence there is negligible long range interaction between dislocations and radiation-induced defects.