Department of Medicine and Department of Cell and Structural Biology, University of Manchester, Stopford Building, Oxford Road, Manchester m139pt *Department of Clinical Biochemistry, Clinical Sciences Building, Hope Hospital, Eccles Old Road, Salford m6 8hd received 14 April 1988 Introduction An understanding of the mechanisms underlying the differentiation of mammalian cells represents one of the most challenging aspects of modern cell biology, and the success of investigations into such processes has been greatly influenced by the availability of experimental in-vitro systems in which the cellular environment closely resembles that encountered by the cells in vivo. For any particular cell type, however, the successful implementation of such experimental models is also dependent upon the isolation of long-term, stable cell lines having a normal phenotype and the differentiation characteristics of cells within the parent tissue. In the absence of a stable, characterized line of thyroid epithelial cells, early studies of thyroid