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Currently, many laboratories that measure plutonium and uranium concentration by isotope dilution mass spectrometry (IDMS) use Large Size Dried (LSD) spikes, which contain both plutonium and uranium in individual glass vials. The overall measurement uncertainty for IDMS depends on the uncertainties associated with the standard reference materials used, as well as on the steps in the measurement procedure, such as weighing. The application of LSD spikes has many advantages due to their stability and due to simplified measurement procedure. Most of the LSD spikes prepared at inspection laboratories and commercially supplied were initially aimed at accurate measurements of the "input solution for reprocessing" with Pu to U ratios of about 1/100 to 1/140. They were prepared by dissolving metal standards and mixing them to contain a few mg of Pu and 40 to 50 mg of U in individual vials. However, MOX powders and pellets, for which the Pu to U ratio ranges from approximately 1:1 to 1:3, are the main samples at the Plutonium Fuel Development Center (PFDC) of JAEA and the LSD spikes aimed to measure input solution contains too much U to measure these MOX samples accurately. Thus, PFDC has prepared LSD spikes for MOX samples with technical support from the IAEA and has prepared several types of LSD spikes with different Pu/U ratios in accordance with sample compositions. On the other hand, while the needs of Pu reference materials and LSD spikes are increasing in Japan along with the development of the nuclear fuel cycle, difficulties of importing these reference materials are also increasing. Securing a stable supply of Pu reference materials for accountancy analysis is essential for facility operation. It is therefore considered important to acquire techniques for the preparation of domestic Pu reference materials in Japan. Available MOX powder with Pu isotopic composition suitable to be used as feed material for LSD spikes was dissolved and the resulting Pu nitrate solution (MOX-Pu) was purified. Spikes to measure MOX samples at PFDC were prepared and practical tests were conducted under collaboration work with Japan Nuclear Fuel Limited (JNFL). Experience in the preparation and utilization of LSD spikes for MOX materials, the results of some practical tests, as well as the certification method for MOX-Pu will be explained in this paper. [ABSTRACT FROM AUTHOR]