The mechanisms underlying the cytotoxic action of pure fullerene suspension (nano-C₆₀) and water-soluble polyhydroxylated fullerene [C₆₀(OH)_n] were investigated. Crystal violet assay for cell viability demonstrated that nano-C₆₀ was at least three orders of magnitude more toxic than C₆₀(OH)_n to mouse L929 fibrosarcoma, rat C6 glioma, and U251 human glioma cell lines. Flow cytometry analysis of cells stained with propidium iodide (PI), PI/annexin V–fluorescein isothiocyanate, or the redox-sensitive dye dihydrorhodamine revealed that nano-C₆₀ caused rapid (observable after few hours), reactive oxygen species (ROS)–associated necrosis characterized by cell membrane damage without DNA fragmentation. In contrast, C₆₀(OH)_n caused delayed, ROS-independent cell death with characteristics of apoptosis, including DNA fragmentation and loss of cell membrane asymmetry in the absence of increased permeability. Accordingly, the antioxidant N-acetylcysteine protected the cell lines from nano-C₆₀ toxicity, but not C₆₀(OH)_n toxicity, while the pan-caspase inhibitor z-VAD-fmk blocked C₆₀(OH)_n-induced apoptosis, but not nano-C₆₀–mediated necrosis. Finally, C₆₀(OH)_n antagonized, while nano-C₆₀ synergized with, the cytotoxic action of oxidative stress–inducing agents hydrogen peroxide and peroxynitrite donor 3-morpholinosydnonimine. Therefore, unlike polyhydroxylated C₆₀ that exerts mainly antioxidant/cytoprotective and only mild ROS-independent pro-apoptotic activity, pure crystalline C₆₀ seems to be endowed with strong pro-oxidant capacity responsible for the rapid necrotic cell death.