For further characterization immunoglobulins G (IgG) of 14 patients with Graves' disease were fractionated on Protein A bound Sepharose. Sufficiently enriched IgG-subclasses were obtained despite incomplete separation. TSH displacing activity (TDA) was not homogeneously distributed in the subclasses, however IgG1 was most predominant in TDA followed by IgG3, IgG2 being essentially inactive. In two subjects TDA in whole IgG gave positive results in subclasses after fractionation, thus providing evidence, that it would be theoretically possible to move sensitivity of the method close to 100% after subfractionation. TDA was not enriched together with thyroid cell antibodies. We conclude from these results that TDA-IgG is characterized by restricted heterogeneity notwithstanding the incomplete separation of the IgG-subclasses.