The long-acting thyroid stimulator (LATS) detectable in the serum of some patients with thyrotoxicosis is associated with immunoglobulin G(IgG) (Kriss, Pleshakov & Chein, 1964). Moreover, recent studies indicate that the thyroid-stimulating activity is an inherent property of specific IgG molecules. The distribution of activity within the molecule parallels antigen binding activity in antibodies (Dorrington, Munro & Carneiro, 1964; Dorrington, Carneiro & Munro, 1965) and it has been suggested that LATS may be an autoantibody to a thyroidal antigen (Kriss et al. 1964; Adams, 1965; Dorrington et al. 1965). Preliminary characterization of this hypothetical antigen has been attempted by studying the effects of absorption of LATS-IgG with thyroid cell-fractions. Absorption of pituitary thyroid-stimulating hormone (TSH) under identical conditions was also studied. Preparations. The isolation of immunochemically pure LATS-IgG from active sera has been described previously (Dorrington et al. 1965). Human pituitary TSH (20 m-u./mg.) was prepared by the method of Steelman,