Three sensitive assays (Behring, Organon and LKB-Pharmacia) for measurement of serum thyrotropin (TSH) were evaluated. All three assays showed good precision, sensitivity, linearity and for practical purposes negligible high dose hook effects. The correlations between the assays were excellent, but due to standardisation and/or matrix effects there were incomplete recoveries in two kits and the kits showed systematic differences of up to 20% in the TSH values obtained. The clinical utility of the three assays was investigated in patients with thyroidal and non-thyroidal illnesses and in healthy subjects with thyroxine binding abnormalities. According to the results of the analytical and clinical evaluation, all three kits are acceptable and reliable as first-line thyroid function tests.