Cellular hypersensitivity was studied using in vitro lymphocyte transformation technique with PHA and indirect MIF test in 53 patients with Hashimoto's thyroiditis, 29 patients with Basedow's disease and in 16 normal subjects. Lymphocytes were separated from peripheral blood by cotton columns. In the studies of lymphocyte blastgenesis, 2ml of lymphocyte suspension containing 1.5×10(6) cells/ml in TC-199 supplemented with 15% calf serum were cultured with 200μg PHA for 72 hours at 37℃ in an atmosphere containing 95% air and 5% CO2, and blastoid cells were identified by microscopic examinations. For the MIF test, 2ml of 3×10(6) lymphocytes/ml in TC-199 containing appropriate antigens (human thyroid microsome fraction; 100-500μg wet weight/ml, thyroglobulin; 100μg/ml) were cultured at the same circumstances for 72 hours and then the culture supernatants were examined for the migration of guinea-pig peritoneal macrophages. Migration index (M. I.) was a rate of the area of migration in supernatant with antigen to that without antigen.The rate of blastogenesis of lymphocytes in response to PHA was 32.5±11.2% (mean±SD) in 26 patients with Hashimoto's disease, 49.0±7.6% (mean±SD) in 11 patients with Basedow's disease, and 49.0±8.1% in 14 normal subjects. Significant low response to PHA (the rate under 33%, mean-2SD of normal subjects)was recognized in 14 patients (54% ) with Hashimoto's disease. No patient with Basedow's disease showed abnormal response to PHA. Migration index (MI) with thyroid microsomal fraction was 85.6±19.5% in 40 patients with Hashimoto's disease, 96.6±8.2% in 15 patients with Basedow's disease, and 97.6±10.1% in 18 normal subjects. MI under 77.4% (mean-2SD of normal subjects) was recognized in 12 patients (30%) with Hashimoto's disease. But, MIF production against native or denatured thyroglobulin was negative in all patients examined. There was no definite correlation between MIF production in response to thyroid microsomal fraction and antithyroglobulin antibody titer in patients with Hashimoto's disease.But the patients with positive MIF test showed low or negative thyroglobulin antibody in serum. There were no appearant correlations between MIF test, blastogenesis of lymphocytes with PHA and clinical findings such as age, duration of disease, histology and thyroid function. Above results indicated that T-lymphocyte activity or population of T-lymphocytes responding to PHA was decreased in Hashimoto's disease. On the other hand, it was also shown that the population of lymphocytes sensitized against thyroid autoantigens was present in the peripheral circulation of some patients with this disease. Whether they were T-or B-lymphocytes was not conclusive from the present studies. These sensitized lymphocytes might play important roles in the pathogenesis of Hashimoto's thyroiditis.