This study was concerned with the localization of the facteur thymique sérique (FTS) by immunoelectronmicroscopy in cultured thymic epithelial cells. The FTS antibodies labelled differently sized cytoplasmic vacuoles containing a substance of variable density. This substance occurs occasionally in crystalline structures which reacted strongly with purified FTS antibodies. On the ultrastructural level the FTS-positive cells were demonstrated to fix specifically keratin antibodies and to contain tonofilaments. Some of these cells originated from the thymic medulla proved by the presence of microvilli-forming cytoplasmic vacuoles. The existence of FTS-positive crystalline inclusions confirmed the hypothesis of a storage process in the cytoplasm of thymic epithelial cells in vitro or in pathological conditions.