It has been demonstrated that fast neutron irradiation produces significant hardening in simple Fe-(3-18)Cr binary alloys irradiated to about 35 dpa in the temperature range 365 to 420°C, whereas irradiation at 574°C produces hardening only for 15% or more chromium. The irradiation-induced changes in tensile properties are discussed in terms of changes in the power law work hardening exponent. The work hardening exponent of the lower chromium alloys decreased significantly after low temperature irradiation (< 420°C) but increased after irradiation at 574°C. The higher chromium alloys failed either in cleavage or in a mixed ductile/brittle fashion. Deformation microstructures are presented to support the tensile behavior.