A cationic carotenoid derivative (GRP-carotenal) was synthesized by the reaction of Girard's reagent P and β-apo-8′-carotenal. The singlet-oxygen quenching constants for GRP-carotenal were 1.3 ± 0.1 × 10¹⁰ and 1.0 ± 0.1 × 10¹⁰ M⁻¹ s⁻¹ in acetonitrile and in detergent micelles, respectively. Photosensitized damage to K562 leukemia cells from cis-di(4-sulfonatophenyl)diphenylporphine, hypericin and protoporphyrin IX was inhibited by GRP-carotenal under conditions where β-apo-8′-carotenal, β-carotene and crocetin were ineffective. The unique cytoprotective properties of GRP-carotenal, relative to the other carotenoids studied, could not be explained by the differences in the cell content of the various carotenoids or by the changes in the cell content of the photosensitizers used. Photosensitizer fluorescence from labeled K562 cells was reduced by GRP-carotenal but not by the other carotenoids studied. The novel photoprotective properties of GRP-carotenal may be due to its subcellular distribution. In photosensitizer-containing detergent micelles, novel properties of GRP-carotenal were not apparent. None of the carotenoids studied reduced photosensitizer fluorescence or singlet-oxygen generation. Singlet-oxygen quenching by GRP-carotenal and by β-apo-8′-carotenal were roughly the same. Crocetin has a singlet-oxygen quenching constant that is about a factor of five lower. Singlet-oxygen quenching by β-carotene was limited by its aggregation.