Fullerene (C60) is known to efficiently generate singlet oxygen when irradiated with light. In spite of such an unique photodynamic nature, little has been studied on biological functions of C60. This paper describes a tumor-therapeutic trial of C60 based on the photo- induced generation of active oxygen. To achieve preferential accumulation of C60 in the tumor tissue, this water-insoluble drug was chemically conjugated with poly(ethylene glycol) (PEG) not only to make it soluble in water but also to make the molecular size larger. When intravenously injected to tumor-bearing mice, the large-sized, as expected, water-soluble C60- PEG conjugate was accumulated to a greater extent and retained for a longer time period in the tumor tissue than in the normal issue. Following intravenous injection of the C60-PEG conjugate, local irradiation of visible light to the tumor site induced tumor necrosis, in contrast to the conjugate injection alone. When the C60 dose was 8.48 mg/mouse, the tumor mass completely disappeared at an irradiation power of 107 J/cm2, indicating a high potentiality of the C60-PEG conjugate for photodynamic tumor therapy.