Abstract: The amount of the residual plutonium-239 in the nuclear explosion debris was determined by mass-spectrometric and alpha-spectrometric analyses. The error caused by the influence of neptunium-239 on the determination of plutonium-239 was corrected by di-:rect subtraction or linear extrapolation. A radiochemical analytical procedure was proposed and a method for storing the plutonium isotopic diluent of very low concentration (10-9M) was recommended. An error less than±5% could be obtained for the direct subtraction method which is applicable only when the total yield of uranium-239 is small, and when using this method, the uranium and neptunium must be separated from plutonium as early as possible. The error of the linear extrapolation is generally less than±4%, and this method is superior to the former one when the total yield of uranium-239 is large.