The processing of a unique 8-oxoguanine residue in DNA has been studied in mammalian cells using a single-stranded shuttle vector. A fragment of human Ha-ras carrying the lesion on the first (G1) or the second guanine (G2) of codon 12 was inserted in a shuttle plasmid. Extrachromosomal DNA is replicated in animal cells, extracted and used to transform bacteria to be amplified and individualized. DNA sequencing of bacterial clones showed the mutagenic potency of 8-oxoguanine in vivo to be approximately 4%. The presence of the 8-oxoguanine does not greatly affect survival of the progeny. No significant difference was observed between the mutation frequencies induced by 8-oxoguanine located either at the G1 or G2 position. The majority of the mutations, targeted at the lesion level, are G to T transversions. These base substitutions induced respectively glycine to cysteine (G1) or valine (G2) change in the P21ras protein. These mutations may contribute to activation of the protooncogene, leading to spontaneous tumorigenesis.