The present study focuses on the establishment and characterization of a new follicular thyroid carcinoma cell line. The human cell line ML-1 was derived from a dedifferentiated follicular thyroid carcinoma relapse, which progressed despite preceding surgery followed by two radioiodine therapies. More than 90% of the cells of this line express thyroglobulin, chondroitin sulfate, and vimentin antigens, but only about 70% show cytokeratin filaments and a negative surface charge density such as human erythrocytes. More importantly, cells of this line are able to take up iodine and/or glucose both in vitro and in vivo and to secrete thyroglobulin, chondroitin sulfate, and fibronectin into the interstitial space. In addition, triiodothyronine is released constitutively into culture supernatants. Moreover, it is also suitable for xenotransplantation studies because it is tumorigenic in NMRI nude mice in vivo. The cell line forms tumors with follicular structures when transplanted to nude mice. Due to these unique features the ML-1 cell line can be considered as a very suitable test model for pharmacological and cell biological studies. Since chemicals may interfere with the production of thyroid hormones, this cell line represents also a tool for toxicological investigations.