The effect of reactor irradiation upon the notched-bar impact properties of several mild-carbon steels was investigated. ASTM types A 70³ and A 212⁴ boiler steels, and aluminum treated, 0.34 per cent carbon steel were tested. The ductile-to-brittle transition temperature was raised significantly by irradiation and the maximum impact strength, under ductile conditions, was reduced in some cases.