An inherited defect of suppressor T-lymphocytes has been hypothesized in Hashimoto’s thyroiditis. To assess this hypothesis, human T-lymphocyte subsets (TG, T lymphocytes with surface receptors for the Fc fragment of immunoglobulin G; TM, T lymphocytes with Fc receptor for immunoglobulin M) have been studied in nine patients affected by the disease. TG cells have been previously shown to be suppressors in the poke weed mitogen-stimulated B-cell differentiation and have proved abnormal in several autoimmune or immunodeficiency disorders. The number of TG lymphocytes in the patients did not differ from that in normal controls. It is possible that 1) suppressor Tlymphocytes are not involved in the pathogenesis of Hashimoto’s disease or 2) antigen-specific suppressor T-cells are involved, but too are low in number with respect to total TG.