We have demonstrated that blue light has anticancer effects in cultured cancer cells and tumor-bearing animals. Based on our experimental findings, in addition to cytostatic activity that suppresses the proliferation of B16 melanoma cells, blue light may exert cytocidal activity through interaction with vitamin(s) contained in the culture medium. The present study was undertaken to identify the specific vitamins with which blue light interacts and to investigate the factors responsible for its cytocidal activity. B16 melanoma cells were incubated in media supplemented with various vitamins and exposed to blue light for 10 min. Cell necrosis was observed only in media containing riboflavin (0.4 mg/l). The effects of other components of visible light on riboflavin were also studied. Riboflavin-containing media were exposed to light of each of the three primary colors (red, green and blue) and the effects on the colony-forming capacity of B16 melanoma cells were evaluated. Cell necrosis was induced only in media exposed to blue light. The effects of riboflavin increased in a concentration-dependent manner in the range from 0.3 to 1.0 mg/l in blue-light-exposed media and were antagonized by the presence of catalase (200 U/ml). These findings suggest that cell necrosis is probably induced by active oxygen species such as hydrogen peroxide formed by the reaction of riboflavin with blue light.