A highly sensitive radioimmunoassay for the determination of thyroxine in blood spotted on filter paper is described. The radioimmunoassay developed by Weeke and Ørskov for determination of serum thyroxine was simplified and shortened, and its cost was decreased, for use in programmes for mass screening of thyroid function. The method permits the use of small (2 mm diameter) dried blood dots, thus avoiding interference by binding proteins. Polyethylene glycol 6000 is used for the final separation of antibody bound labelled thyroxine, instead of the expensive and time-consuming wick chromatography used in the reference method. Serum thyroxine concentrations obtained using dried blood by the modified procedure correlate well with those obtained using serum by the reference method. The distribution of serum thyroxine concentration for a population of 2134 babies is given.