A sensitive thyroxin /T4/ radioimmunoassay procedure with a short incubation time /10 min/ similar to a conventional stat test in clinical chemistry is described. The assay parameters such as concentration of antisera and125I-T4, and incubation temperature that affect the kinetics of antigen-antibody reaction have been studied. Within- and between-assay variations were less than 7% CV. Analysis of 45 serum samples by the proposed method and by a conventional assay gave similar results /Y=1.05X–0.187; r=0.990/.