Oncogenic transformation has been induced in vitro in the C3H/10T1/2 clone 8 line of mouse cells by exposure to 5-fluoro-2'-deoxyuridine (FUdR) or 5-fluorouracil. This transformation is both dose and time dependent and can be markedly decreased by simultaneous exposure of the cells to thymidine. The transformation induced by 5-fluorouracil is probably due to its intracellular conversion to FUdR or its monophosphate. Transformation by FUdR was found to be cell cycle dependent with maximum sensitivity to transformation occurring in early S phase. Cell lines that produced sarcomas in antithymocyte-treated syngeneic mice were isolated from FUdR-transformed cultures. Trifluorothymidine, 5-bromo-2'-deoxyuridine, and 5-iodo-2'-deoxyuridine induced no transformed foci in the C3H/10T1/2 clone 8 cell line. Thus, not all mutagens produce oncogenic transformation nor does the lack of mutagenicity, as classically measured, completely exclude the possibility that a given agent is oncogenic. Also, there was no evidence of the "switch on" of oncornaviral information in the FUdR-transformed cell lines.