It has been shown that LATS activity is mainly distributed in the fraction of immunoglobulin G (IgG) in the serum from hyperthyroid patients. The present paper examined the immunological character of LATS and the method for separation of LATS activity from LATS positive serum using DEAE-Cellulose and affinity chromatography methods. LATS activity was distributed in the IgG fraction that could be separated by the DEAE-Cellulose column equilibrated with a 0.0175 M prosphate buffer, pH 6.3 from LATS positive serum. When LATS positive serum was fractionated by affinity chromatography on a Sepharose-bound antibody against human IgG, Fab of IgG and Fc of IgG, LATS activity was always retained in IgG fraction. When LATS positive serum was fractionated by affinity chromatography on a Sepharose-bound anti-K chain, LATS activity was found in the fraction that reacted with the anti-K chain. Because of the low antibody titer of the anti-lambda chain, LATS activity did not react with this antibody. By affinity chromatography on Sepharose-bound Concanavalin A, serum LATS activity was also retained in IgG fraction. LATS activity could be separated from LATS positive serum without significant loss of biological activity by affinity chromatography. When IgG (1) was purified from the fraction by affinity chromatography on anti-IgG (1)-bound Sepharose, about 80% of the original LATS activity was found in IgG (1) fraction. When the Fab fragment of IgG (1) was separated from papain hydrolysed IgG (1), using a Protein A-bound Sepharose column, a short-acting type of thyroid stimulating activity was found in only this fraction. These data suggest that the biological activity of the thyroid stimulation is distributed mainly in the Fab fragment of IgG (1).