The input end of a reprocessing plant is the first point in the fuel cycle where the plutonium produced in reactors can be accurately measured. The current practice for measuring the plutonium entering a reprocessing plant is to determine the total amount of this element in each batch in an accountability tank by the volume concentration method. This involves the measurement of the concentration of plutonium in the sample and the volume, density, and temperature of the solution in the tank; each of these measurements contributes to the total error in the input accountability measurement. Other approaches being studied are the Pu/U ratio method and the isotope correlation technique. These depend heavily on data from the fabrication plant and the reactor and require a good estimate of the losses through hulls. Through developing tracer techniques for the input accountability of plutonium in reprocessing plants, two tracers have been identified and tested: magnesium and lead. The corresponding techniques have been named MAGTRAP (Magnesium Tracer technique for the Accountability of Plutonium) and LEADTRAP (Lead Tracer Technique for the Accountability of Plutonium). The method involves the addition of a known amount of tracer to the input accountability tank and the subsequent measurement of the plutonium-to-tracer ratio in a sample of the tank solution. By knowing the amount of tracer element added, the total amount of plutonium in the tank can be obtained. The validity of this technique has been established by a series of experiments in the input accountability tank of a reprocessing plant. Accuracies of better than 1% are attainable by this method.