The presence of thyroid hormone autoantibodies was investigated in a 48-year-old non-goitrous woman with high levels of serum total T3 and free T3. Her other thyroid function tests were normal. The presence of thyroid hormone autoantibodies were assessed in serum, acid-charcoal treated serum, IgG fractions and in a purified IgG fraction. The IgG fraction was separated from serum by a protein A-Sepharose CL-4B column chromatography and purified by gel filtration chromatography using Sephacryl S 200. Sera from normal individuals were used as controls. The results showed that the increased serum total and free T3 levels were caused by the presence of anti-T3 IgG autoantibodies. The results also indicate that protein A column chromatography is useful as a screening method for gross qualitative analyses of thyroid hormone autoantibodies.