Thyroid-stimulating immunoglobulins were prepared from two potent sera, one contained long-acting thyroid stimulator (LATS) and the other contained both LATS and LATS-protector (LATS-P). The potencies of the immunoglobulin G (IgG) preparations were estimated in the McKenzie assay. The accumulation of cyclic AMP in mouse thyroid lobes was stimulated only by LATS–IgG; LATS-P–IgG was inactive. In contrast, both LATS–IgG and LATS-P–IgG were equally effective in slices of human thyroid.