Porcine thyroid epithelial cells, isolated and cultured on a type I collagen substrate, formed a monolayered sheet. When the apical surface of the monolayer was further covered with the same collagen substrate, the epithelial cells migrated and formed intercellular follicular lumen-like structures. In addition, a true intracellular follicular lumen sometimes appeared, especially in cells of the monolayer region. It was proved by serial sections that the intracellular follicular lumina, 1-5 microns in diameter, lined by rather long microvilli and stained for PAS, had no openings to the extracellular space. The intracellular lumina are presumed to fuse with each other to form an intercellular follicular lumen, or fuse with the performed intercellular follicular lumen. Auto-radiographic studies using 125I revealed that iodination of thyroglobulin takes place in the peripheral region of both intracellular and intercellular follicular lumina.