The assay of a light water reactor rod for its fissile uranium content can be accomplished by irradiating the rod in a water bath with neutrons from an antimony-beryllium source. The different slowing down properties of the moderator for neutrons of different energy cause a much stronger attenuation of the source neutron flux than of the flux originating from the induced fissions. At 25 cm from the source, the fission neutrons already contribute 20% to the signal measured by ordinary thermal-neutron detectors, allowing determination of their intensity as a measure of the fissile content of the rod. Laboratory experiments demonstrated that a 30-mCi source is sufficient to attain 1% precision in 1 min.