Light is known to interact with endogenous or exogenous chemical agents in the skin or eyes, to produce photosensitization. This phenomenon may be deleterious (phototoxicity or photoallergy) or may be useful therapeutically (photodynamic treatment of psoriasis or tumors). The objective of this project is to determine the role played by light-induced free radical species in photosensitization. In addition, efforts are directed towards the identification of photosensitizers that may be more efficient therapeutically. Disperse blue 35 is an anthraquinone-based dye mixture which causes photocontact dermatitis in factory workers. The main component of the dye, 4, 5-diamino-1, 8-dihydroxyanthraquinone, was found to be a potent generator of both singlet oxygen and superoxide upon visible light irradiation. UV-irradiation of another photosensitizing anthraquinone-derived dye benzanthrone (7H-benz[de]-anthracene-7-one) resulted in the generation of both singlet oxygen and super oxide in high yield. Active forms of oxygen were also implicated in the photo killing of gram-positive bacteria by curcumin. 1,5-Diamino-4,8-dimethoxy-anthraquinone and related analogs were efficient generators of both superoxide and singlet oxygen upon visible light illumination. These compounds were active against K562 human chronic myeloid leukemic cells in culture causing frank strand breaks in DNA. Anthrapyrazoles, which are also potential photodynamic agents, become strong oxidizers upon illumination with visible light.