The possible consequences of depurination for both spontaneous and induced mutagenesis were investigated using in vitro and in vivo assays. Depurination of synthetic polynucleotide templates such as poly [d(A-T)] or poly [d(G-C)] leads to increased misincorporation of noncomplementary nucleotides when these templates are copied by prokaryotic and eukaryotic DNA polymerases. The ability of Escherichia coli DNA polymerase I to copy over apurinic sites was demonstrated using single-stranded circular DNA of bacteriophage 0X174 as a template and starting DNA synthesis at a fixed point. Analysis of the newly synthesized 0X174 restriction fragments on neutral and alkaline sucrose gradients shows that synthesis proceeded past apurinic sites. When using depurinated 0X174 DNA containing the am3 amber mutation as a template for copying by E. coli DNA polymerase I, an increased reversion to wild type is observed after transfection into E. coli spheroplasts. The enhancement in reversion frequency is proportional to the extent of depurination, suggesting that depurination is also mutagenic during copying natural DNA in vitro. When noncopied depurinated 0X174 am3 DNA is transfected in E. coli spheroplasts, no increase in reversion frequency is observed above background level. However, when the spheroplasts are derived from bacteria in which the SOS response had been induced by UV irradiation, a substantial increase is observed for depurinated molecules, whereas no increase is observed for nondepurinated templates, suggesting in vivo mutagenesis at depurinated sites. In each of the different assay systems investigated, the increase in misincorporation or reversion frequency is a linear function of the number of sites and is abolished by treatment of the depurinated templates with alkali, which rapidly induces strand breakage at apurinic sites.