The investigation of the tested commercially available T4-RIA-Kits have shown that: The separation of hypo-, eu- and hyperthyroid values is better with kits a) and f) than with b),c),d) and e), though all kits give acceptable values under routine operating conditions, kit b) takes less time and work than the other tested kits, depending on the kit used, the time needed for 100 determinations varies between 1,5 and 4 hr. A general recommendation for one of the tested T4-RIA-Kits cannot be made since the choice of a kit depends not only on the criteria discussed but also on local circumstances.