The type-II photosensitization process is mediated by the formation of singlet oxygen (O₂[Δ_g]). The short lifetime of this species dictates that chemical reactions with biological substrates can only occur when O₂(Δ_g) is in very close proximity to the photosensitizer itself. In this study, deuteroporphyrin, a type-II, membrane-localized photosensitizer, was used to generate O₂(Δ_g) in human lymphoblast WTK-1 cells, and the range of influence was determined by a variety of biological assays. Surprisingly, the initial membrane-confined events were shown, by comet assay, to induce DNA damage in these cells. DNA damage was inhibited both by membrane-localized (α-tocopherol acetate) and by cytoplasmic (trolox) free radical scavengers. Comet formation also was inhibited by treatment at low temperature. DNA fragmentation was not influenced by treatment with the pan-caspase inhibitor, benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone, showing that apoptosis was not responsible for fragmentation. Taken together, these results show that primary photosensitization reactions involving O₂(Δ_g), even when tightly confined in extranuclear locations, leads to the production of secondary reactive oxygen species, probably as a result of lipid peroxidation, that can act at greater distances from the photosensitizer itself. These experiments were carried out under conditions where cell survival was significant and raise questions regarding DNA damage and mutagenesis pathways, even when extranuclear O₂(Δ_g)-generating compounds are used.