Fischer rat thyroid epithelial cella (FRT) growing continuously in culture were infected with the Kirsten murine sarcoma virus KiMSV(KiMuLV) and found to produce this virus constitutively. Although the morphology of the FRT cells did not change appreciably, the cells became malignant after infection with KiMSV(KiMuLV) as shown by the growth of infected cels in semi-solid media (uninfected FRT cells did not grow) and by the tumorigenicity of infected when injected into syngeneic animals (uninfected FRT cells or FRT cells infected with non-transforming retro-viruses were not tumorigenic). The induced tumors morphologically resembled moderately differentiated carcinomas. Two markers of thyroid epithelial differentiation were absent in the original FRT clone and remained unexpressed after transformation. Fully differentiated rat thyroid epithelial cells (FRT-L cells) infected with another strain of the Kirsten murine sarcoma virus, the KiMSV(MolMuLV), were also transformed as demonstrated by the ability also of these cells to grow as carcinomas (after in vitro transformation) in syngeneic animals. Our results clearly demonstrate that the Kirsten murine sarcoma virus can transform in vitro cells of epithelial as well as of fibroblastic origin.