- The present study was carried out with a normal rat kidney cell clone that was initiated from a single cell infected by only one infectious particle of the non-transforming retrovirus, Moloney murine leukemia virus. Although the parental cells were highly resistant to chemical carcinogenesis, this infected clone was rendered susceptible to transformation by chemical carcinogens. However, when it was exposed to the tested carcinogen at a low subculture passage post-infection, cell transformation was detectable only after many subsequent passages. On the other hand, if it was exposed to the carcinogen at a high passage post-infection, cell transformation was detectable sometimes even without further passage, or in other cases after the first subsequent passage. Mouse interferon could inhibit the transformation by carcinogen employed at the low but not at the high passage post-infection. This inhibitory effect was reversible; if interferon was removed, even after the cells had been passaged many times in its presence, cell transformation became visible at passage 11 after interferon removal, although no treatment with the carcinogen was repeated. Interferon had no effect on the replication or the focus-forming capacity of cells transformed by such chemical-retroviral co-carcinogenesis. The possibility that this carcinogenic process depends on amplification of the integrated provirus DNA, and that this amplification can be inhibited by interferon is discussed.