Subpopulations of human peripheral blood T lymphocytes were examined in twenty-three myasthenic patients. T lymphocytes bearing receptors for the Fc portion of IgG (T gamma) were significantly increased in a third of the patients examined. T lymphocytes bearing receptors for the Fc portion of IgM (Tmu) were within normal values in all but two patients. Possible implications of these cells in the pathogenesis of myasthenia gravis are discussed.