1. A pure strain of thyroid epithelium was isolated and maintained in active condition for 7 months. At the end of the experiment, the rate of cell multiplication was as great as at its beginning.
2. The thyroid cells grew at the surface of the coagulum as pavement epithelium, and within the coagulum as a glandular structure.
3. The cells did not dedifferentiate, and the lumen of the acini in cultures from a strain over 4 months old contained colloid secretion similar morphologically to that from a freshly extirpated thyroid gland.