After treatment of poly(dC) by the simple alkylating agent (3H)dimethylsulfate, 90 per cent of the radioactivity cochromatographied with 3-methylcytosine and 10 per cent with 5-methylcytosine which is the normally occurring methylated base. In order to study the influence of 3-methylcytosine on DNA replication, untreated and DMS-treated poly(dC) were used as templates for E. coli DNA polymerase I. The alkylation of poly(dC) inhibits DNA chain elongation, and does not induce any mispairing under high fidelity conditions. The alteration of DNA polymerase I fidelity by manganese ions allows some replication of 3-methylcytosine which mispairs with either dAMP or dTMP. Our results suggest that 3-methylcytosine could be responsible, at least partially, for the killing and the mutagenesis observed after cell treatment by alkylating agents.