The principle of the liquid and solid two-phase radioimmunoassay and its application to measuringthe concentrations of triiodothyronine and thyroxine of human serum in a single sample at the sametime are described in this paper. Rabbit anti-triiodothyronine antiserum was immobilized on the innersurface of a plastic tube as solid-phase radioimmunoassay system of testing triiodothyronine and donkeyanti-rabbit gamma globulin antiserum was used to separate immunocomplex in liquid-phase radioim-munoassay system of thyroxine reaction. In both systems, radioiodine-125-labelled antigens were usedas tracer. Through simple calculation, we can ger the results of serum concentrations of triiodothyro-nine and thyroxine in a single sample. Compared with classical single-component radioimmunoassay oftriiodothyronine and thyroxine and by methodological appraisal of two-phase radioimmunoassay itself,this method has the advantages of very high efficiency, reliability and also has the same specificityand precision. The principle of the liquid and solid two-phase radioimmunoassay and its application to measuring the concentrations of triiodothyronine and thyroxine of human serum in a single sample at the same time are described in this paper. Rabbit anti-triiodothyronine antiserum was immobilized on the innersurface of a plastic tube as solid-phase radioimmunoassay system of testing triiodothyronine and donkeyanti-rabbit gamma globulin antiserum was used to separate immunocomplex in liquid-phase radioimmunoassay system of thyroxine reaction. In both systems, radioiodine-125-labeled antigens were used as tracer. Through simple calculation with we can ger the results of serum concentration of triiodothyro-nine and thyroxine in a single sample. Compared to classical single-component radioimmunoassay oftriiodothyronine and thyroxine and by methodological appraisal of two-phase radioimmunoassay itself, this method has the advantages of very high efficiency, reliability and also has the same specificityand precision.