Treatment of mice with methylthiouracil to produce enlarged, hyperplastic thyroid glands was used to facilitate testing whether LATS protector would react with mouse thyroid extract in vitro. It did not. As LATS protector injected into mice shows no thyroidstimulating activity and as it does not block the action of LATS in vivo, there is thus no evidence that LATS protector reacts with mouse thyroid at all. LATS protector was found in all of 20 thyrotoxic patients not showing LATS. This evidence suggests that LATS protector is a human thyroid-stimulating autoantibody, which differs from LATS in that it does not cross-react with the mouse.