A research methodology concerning introduction of thyroxine radioimmunoassay applicable to screening programs is reported. The reagents necessary for T4 assay were prepared and tested and the technical parameters, the stages and technical characteristics of RIA-T4 in the laboratory were established. The RIA method for T4 was compared with the CPBA method and with the commercially available kits, both on samples taken from experimental animals and on hypo-, hyper- and normo-thyroid subjects. The range of T4 normal values could thus be established for the geographic area of Romania: 7.5-10.5 microgram dl-1, with a mean of 9.1 +/- 2 SD. The reagents were applied by extension to thyroxine assay on dry blood spots. The results are comparable with those of direct serum assay. The value of these assays in screening thyroid function in newborns and in the population with remnant thyreopathic endemic dystrophy is discussed.