Peripheral lymphoid cells from euthyroid and hyperthyroid subjects were tested for proliferative responses to human thyroglobulin in vitro using [3H] thymidine incorporation as assay. Non-fractionated lymphoid cells from both groups displayed weak stimulation. Using a rosette technique for separating sub-populations of lymphoid cells it was concluded that T-cells constituted the responding cells. The results indicate that both hyperthyroid and euthyroid subjects possess clones of T-cells which are reactive against human thyroglobulin.