The PC Cl 3 cell line is a well characterized epithelial thyroid cell line of Fischer rat origin. This cell line has the peculiarity of retaining in vitro the typical markers of thyroid differentiation (i.e. thyroglobulin synthesis and secretion, iodide uptake and dependence on TSH for growth). The PC Cl 3 cells have been transfected with the E1A gene of Adenovirus 5. The E1A transfected cells, PC E1A, partially lost the dependency on TSH for growth and completely lost the ability to trap iodide and synthesize thyroglobulin; however they did not acquire the typical markers of the neoplastic phenotype. A highly malignant phenotype was achieved after infection of the PC E1A cells with retroviruses carrying the v-raf, v-abl and polyoma virus middle T oncogenes. In contrast, the PC E1A cells transfected with the E1B gene of Adenovirus were not tumorigenic at all, and those infected with retroviruses carrying oncogenes of the ras family displayed a very weak tumorigenic phenotype.