Sera from patients with Graves' disease and Hashimoto's thyroiditis were reacted with normal T lymphocyte preparations in an attempt to detect binding of immunoglobulin G (IgG) to T cells. Sera from normal subjects and patients with toxic adenomas served as controls. Each serum was reacted with at least three different preparations of normal T cells. Bound IgG was identified using a fluoresceinated second antibody, antihuman IgG. Positive cells were enumerated by means of epifluorescent microscopy. IgG from 57.8% of toxic Graves' patients, 30.7% of Graves' patients who were euthyroid after treatment, and 41.6% of Hashimoto's patients bound to normal T cells more than did IgG from normal controls. Reactivity of toxic adenoma sera was similar to that of normal sera. When the positive sera were reacted with helper or suppressor/cytotoxic T cell preparations (separated by negative selection technique), the binding was shown to be directed against suppressor/cytotoxic T cells but not against helper cells. These data indicate that a significant proportion of patients with autoimmune thyroid disease have IgG in their serum which react with a subset of normal T suppressor/cytotoxic cells. This phenomenon could be the expression of anti-lymphocyte antibodies, which may relate to previously recognized reductions in number and function of suppressor T cells in autoimmune thyroid disease.