A radioimmunoassay for triiodothyronine (T3) is demonstrated. Details of the T3 antibody binding reaction are discussed and the statistical data of the assay system are presented. Plasma samples were first extracted by a simple ion exchange method and the extracts then tested by radioimmunoassay. The mean normal concentration was 1.35 ± 0.29 ng/ml. Later, direct measurements in serum samples were performed after partial inhibition of T3 binding to TBG by merthiolate and high pH buffer. After correction for the residual non-specific binding a mean normal plasma level of 1.2 ± 0.3 ng/ml was obtained.