Filter paper discs saturated with dried blood can be used in the Immunotube#{174} solid-phase thyroxine radioimmuno- assay. This assay utilizes polypropylene tubes to which antibody to thyroxine is covalently bound. The filter paper standardsand samples are placed in the tubes, followed by an assay buffer that contains 1125-Iabeledthyroxine and compounds to displace thyroxine from its binding proteins. Afterincubation, boundandfreethyroxineareseparated by aspirating or decanting the disc and buffer fromthetube. Thetest can be usedwith 0.32 cm (1/8 inch)or0.64cm (1/4 inch)discs, and gives quantitative resultsthat correlate well withthoseforserumsamples.The intra-assay coefficient of variation is <10%. The assay may readily be mecha- nized with existing disc-punching equipment, and results of its use in mass screening programs are described. Additional Keyphrases: pediatric chemistry screening thyroid disease . congenital hypothyroid- ism normal values for neonates