An 124Sb-Be(γ,n) assay system has been designed and fabricated for the measurement of fissile content in small samples, fuel rods, and solutions. The instrument uses multiple 4He gas tube detectors to count fast neutrons induced in the assay sample by the 124Sb-Be source neutrons. The 4He detectors are biased to count only neutrons with energies above the 26- and 360-keV source neutrons. Computer calculations were used to optimize the design of the irradiation unit, giving a factor of ∼3 improvement in the induced fission rate. The assay unit has been applied to the measurement of small UO2 samples, pressurized water and boiling water reactor fuel rods, and high temperature gas-cooled reactor fuel.