Abstract— We have determined the deleterious effects of singlet oxygen CO,), generated by thermal decomposition of the water-soluble endoperoxide 3,3'-(1,4-naphthylidene)dipropionate (NDPCX), on plasmid DNA. By following the electrophoretic mobility of DNA on agarose gels, we detected single and double strand breaks induced by treatment with NDPO2 The vector employed was a mammalian shuttle vector and the mutagenic consequences of these damages were investigated, using as mutation target the supF suppressor tRNA gene. A high increase of the mutation frequency, over the background, was observed in plasmids transfected in bacteria or after passage through mammalian cells. Trapping agents and quencher effects and other controls confirm the involvement of 'CK in DNA damage and mutagenicity. These findings indicate that 1O2 can induce DNA lesions which are repaired by an error-prone process in prokaryotic and eukaryotic cells.