[en] Thyroxine T₄ radioimmunoassay (RIA) kit has already been developed in this laboratory. For an efficient diagnosis of thyroid disease, however, it is well known that the T₃ RIA should also be carried out in addition to the T₄ RIA. Accordingly, the development of T₃ RIA kit was urgently desired to match the T₄ kit and to hold a sound domestic supply systems. The high specific activity T₃¹²⁵I about 3,000 μCi/μg T₃) could be obtained by radioiodinating T₂ with chloramine-T, and the labelled product could be stahilized. In the preparation of T₃ free serum, charcoal was eliminated easily from serum by high speed centrifugation, and the resulting T₃ free serum was used for the preparation of T₃ standard serum solutions. RIA buffer system could be improved with the use of 0.025M barbital buffer, pH 8.2, containing 0.1% BSA, 0.5% bovine aamma globulin and 0.02% merthiolate. Antibody titer was increased threefold by using the 0.025M barbital buffer; the titer was 8,000: 1 in the 0.078M borbital buffer, pH 8.6, containing 0.1% BSA and 0.1% NaN₃ while it was 26,000 : 1 in the above described 0.025M barbital buffer. The modified buffer system was also efficient for the use in T₄ RIA since it increased the T₄ antibody titer twofold. When the same buffer system was used in T₃ RIA, no significant difference was observed between the use of HSA and of BSA in so far as 0.5% bovine gamma globulin was added to the buffer, contradicting those in the reference. The resalts indicated that the cost for the preparation of both kits can be saved. Quality guaranteed kits could be prepared by careful control of the assay values in comparing with those of the reference control sera. In consequence, there is not any technical difficulty in routine production. (Author)