[en] A combined method of high-resolution gamma spectroscopy and thermal ionization mass spectroscopy without chemical separation was applied for plutonium assay in pure Pu and U-Pu mixed solutions collected at the Rokkasho Reprocessing Plant, Japan. The relative biases of Pu assay results determined by the combined method to the conventional IDMS are less than 0.01% in the pure Pu and the U-Pu mixed solutions. The combined method can be used as an alternative rapid method for Pu assay as a backup to the conventional IDMS method and K-edge densitometry without any loss of precision and accuracy. (author)