Predominantly cellular labelling was observed in radioautographs of rat thyroid glands fixed by perfusion from the aorta at intervals between 5 and 55s after [(125)I]iodide administration via the aorta. 2. When perfusion was delayed for 2min, or if immersion fixation was used, the labelling was predominantly over the peripheral portion of the follicular lumen, in agreement with the observations of other investigators. 3. The findings support the concept that the initial site of binding of iodine to protein is intracellular, but the nature of this protein has not been established.
