This report presents the software APUS for the determination of isotopic composition and age of plutonium measured with high resolution gamma spectrometry. Spectrum must have been evaluated using a spectrum deconvoluting technique in order to get areas for the relevant full-energy peaks. The program is semi-automatic, which means that the analysis report from a spectrum analysis performed with GammaVision is opened in APUS, which then performs all calculations. The program was validated against results from the software PC/FRAM. The advantage with APUS is that the user does not have to be an expert on a program dedicated for isotopic composition measurements, or analysis, of plutonium.