Single-stranded DNA of coliphage M13mp8 was treated with the oxidizing agent, KMnO4, under conditions that selectively form cis-5,6-dihydro-5,6-dihydroxythymine (thymine glycol). Treatment of DNA with 0.7 and 1.4 mM KMnO4 introduced approximately 200 and 400 thymine glycol residues, respectively, per genome. When these DNAs were used to transform Escherichia coli, it was observed that phage survival was reduced in a dose-dependent manner. In studies designed to investigate the effect of DNA oxidation products on replication in vitro, a complementary 15-mer oligodeoxynucleotide was annealed to the oxidized template and extended when the Klenow fragment of DNA polymerase 1 from E. coli. It was observed that lesions in oxidized DNA strongly inhibited DNA elongation and that DNA synthesis was stopped opposite thymine residues. This is taken as suggestive evidence that the thymine glycol is inhibitory to DNA replication.