Singlet oxygen may be generated in cells by either endogenous or exogenous photosensitizers as a result of exposure to UV or visible irradiation. We have used immuno-spin trapping (Free Radic. Biol. Med. 36: 1214, 2004) to identify the subcellular targets of singlet oxygen generated by rose bengal (RB). Confocal fluorescence microscopy of HaCaT keratinocytes incubated with RB clearly showed that the dye entered the cells and was located mainly in the perinuclear region, probably associated with the Golgi apparatus and endoplasmic reticulum. Previous studies by Wright et al. (Free Radic. Biol. Med.34: 637, 2003) have shown that long-lived protein hydroperoxides (POOH) are present in cells exposed to singlet oxygen-generating dyes. The addition of reducing metal ions such as Cu+ to POOH results in the generation of protein-derived radicals, POO(*) and PO(*), which react with the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) to give relatively stable spin adducts. In order to determine the subcellular localization of the protein-DMPO adducts, we exposed keratinocytes to RB/light exposure and then incubated the cells with Cu+ and DMPO. After staining with antibody against DMPO followed by a secondary Alexa Fluor 488 goat anti-rabbit IgG, the intracellular distribution of protein-DMPO adducts was determined by confocal microscopy. The subcellular localization of the protein DMPO adducts was coincident with that of RB. This approach may provide information on the spatial distribution of singlet oxygen generated in cells.