The effects of long-term treatment with mouse interferon on the phenotype of untransformed C3H/10T1/2 cells and cloned derivatives transformed by methylcholanthrene (MCA) and Simian virus 40 (SV40) were investigated. Continuous presence of interferon induced morphologic reversion, with the development of thick, submembranous filaments in MCA-transformed cells, whereas no morphological effects were detected in SV40-transformed cells. Interferon inhibited the proliferation of all three cell lines and maintained low saturation densities. However, prolonged treatment of MCA-transformed cells with interferon rendered them tolerant toward the anti-proliferative and antiviral activities of interferon although 2-A synthetase activity was induced. Interferon treatment reduced the capacity of both MCA and SV40-transformed cells to form colonies in agar and decreased the tumorigenicity of MCA-transformed but not SV40-transformed cells in mice. These results indicate that the same cell type transformed by different means has different sensitivities to a number of interferon-induced changes in the cell phenotype.