High concentrations of O2-methylthymidine, 3-methylthymidine and O4-methylthymidine (O4-MedT) have little effect on the colony forming ability of cultured Chinese hamster (V79) cells. However, the methylated derivatives enhance the toxic effects of thymidine (dT) at concentrations at which the modified nucleosides alone show no toxicity. O4-MedT was the only modified nucleoside which is incorporated into V79 cell DNA at a detectable level. In the presence of added dT there is an increase in the amount of O4-MedT incorporated. O4-MedT did not induce mutation, either forward or reverse, in V79 cells. The results indicate that O4-MedT is incorporated into DNA as an analogue of deoxycytidine and once incorporated behaves as cytidine during DNA synthesis, in contrast to O4-MedT produced in cellular DNA synthesis, in contrast to O4-MedT produced in cellular DNA by methylating carcinogens which is miscoding and potentially mutagenic.