In orderto study the role of LATS in the pathogenesis of the Graves' disease, the serum activity of the LATS was determined by the bioassay of the modified McKenzie methodl. The subjects examined in the study consisted of 76 individuais including 12 cases of normal control, 54 cases with typical Graves' disease and 10 cases of chronic thyroiditis. The data observed in the patients with the Graves' disease were analyzed in comparison with the clinical features, laboratory findings, and responsiveness to the treatment. The results obtained are as follows:1) None of the subjects which did not have the Graves' disease showed a positive LATS activity, except one case with the chronic thyroiditis. 2) Twenty-two oui of the 54 cases with the Graves' disease showed positive results for LATS(40.7%). The positivity was significantly higher in the patients who had been treated with antithyroid regimen but still showed hyperthyroidisim than in the patients who had not been treated. 3) The activity of LATS was gradually decreased or even became absent as the hyperthyroidism was corrected after the treatment. 4) No significant difference was noticed in age &-sex between the positive and negative groups of LATS. 5) There was no evidence of significant correlation between the LATS activity and clinical features.