The water-soluble fullerene derivative γ-cyclodextrin bicapped C 60 [(γ-CyD)2/C60, CDF0] has several clinical applications, including use as a drug carrier to bypass the blood ocular barriers or a photosensitizer to treat tumors in photodynamic therapy. We have assessed the potential ocular toxicity of (γ-CyD)2/C 60 and its aggregated derivatives induced by UVA and visible light in vitro in human lens epithelial cells (HLE B-3). Cell viability using the MTS assay demonstrated that 2 μM (γ-CyD)2/C60 was highly phototoxic to HLE B-3 cells with UVA irradiation, while no effect was observed in the presence of visible light or when maintained in the dark. In contrast, the aggregated derivative (CDF150) showed neither cytotoxicity nor any phototoxic effect even at 30 μM with either UVA or visible light irradiation. In lens cells treated with (γ-CyD)2/C 60, phototoxicity was manifested as apoptosis. Singlet oxygen production measurement using the EPR/TEMP trapping technique determined that (γ-CyD)2/C60 (CDF0) efficiently produced singlet oxygen. The rate of singlet oxygen production decreased with increased aggregation, with no production by the fully aggregated sample formed after 150 min of heating (CDF150). UVA irradiation of HLE B-3 in the presence of (γ-CyD)2/C60 resulted in a significant rise in intracellular protein-derived peroxides. The singlet oxygen quenchers sodium azide and histidine each significantly protected lens cells against (γ-CyD)2/C60 photodamage, but lutein and Trolox (vitamin E) did not. Clearly, singlet oxygen is an important intermediate in the phototoxicity of monomeric (γ-CyD)2/fullerene. Our results also demonstrate that UVA-blocking sunglasses can limit the ocular phototoxicity of this nanomaterial, while nontoxic endogenous antioxidants like lutein or Trolox cannot provide adequate protection.