[en] An on-line monitor has been developed to assay plutonium in nitric acid solutions. The performance of the monitor has been assessed by a laboratory experimentation program using solutions with plutonium concentrations from 0.1 to 10 g/l. These conditions are typical of the plutonium solutions in an input stream to a plutonium-purification cycle in a reprocessing plant following uranium/plutonium partitioning. The monitoring system can be fully automated and shows great promise for detecting and quantifying plutonium in situ, thus minimizing the reliance on traditional sampling and laboratory-analysis techniques. The total concentration and isotopic abundance of plutonium are determined by measuring the absolute intensities of the low-energy gamma rays characteristics of ²³⁸Pu, ²³⁹Pu, and ²⁴⁰Pu nuclides by direct gamma-ray spectroscopy and computer analysis of the spectral data. The addition of a monitoring system of this type to the input stream of a plutonium-purification cycle along with other suitable monitors on the waste streams and on the product stream provides the basis for a near real-time materials control and inventory system. Results of the laboratory-evaluation program employing plutonium in solutions with isotopic compositions typical of those involved in processing light water reactor fuels are presented. The detailed design of a monitoring cell and detection system is given. The precision and accuracy of the results relative to those measured by mass spectrometry and controlled potential coulometry are also summarized