In this study a strain of differentiated rat thyroid cells in continuous culture was used to detect the presence of thyroid growth promoting immunoglobulins (TGI) in IgG preparations of serum of patients with endemic and sporadic euthyroid goiters. TGI was detected when FRTL5 cells were used in a mitotic arrest assay by costimulating the cells with IgG and a suboptimal dose of TSH. TGI was present in 65 of 71 endemic goter cases and in 9 of 14 sporadic goiter cases.