Two infants were found to have markedly increased TSH levels, 104 and 154 mU/l of plasma, respectively, in a routine screening programme for congenital hypothyroidism. THe recall limit used was 50 mU/l of plasma. On follow-up, both infants were clinically euthyroid and had normal serum T4 and T3. The elevated TSH levels were confirmed only with some commercial radioimmunoassay kits--but not with others. Similar results were obtained in TSH assays of samples from their mothers, who had no other biochemical or clinical evidence of thyroid dysfunction. Both mothers had intense contact with rabbits over long periods. The apparent TSH activity was found to be associated with the IgG fraction. It was neutralized by the addition of normal rabbit serum to the samples and was caused by antibodies to rabbit immunoglobulin. The activity was eliminated from the circulation of both infants with a half-life of approximately one month. Apparently, the heterophilic antibodies were of maternal origin and were transferred to the foetus via the placenta. Infants with so-called transient hyperthyrotropinaemia identified in screening programmes have to be reevaluated to exclude false TSH elevations of this type.