Single-strand breaks of plasmid pBR322 DNA photoinduced by hypocrellin A (HA) in liposomes and by sodium salt of 14-dehydroxy-15-deacetyl-hypocrellin A-13-sulfonate (13-SO 3 Na-DDHA) in aqueous solution have been investigated by agarose gel electrophoresis. Upon irradiation with visible light, DNA damage was observed under oxygen and under argon with both HA and 13-SO 3 Na-DDHA. The mechanisms of DNA damage in liposomes and solution have been studied. In photosensitization by HA in liposomes, the hydroxyl radical .OH is the main agent responsible for DNA damage. During photosensitization by 13-SO 3 Na-DDHA in buffer solution, 1 O 2 and .OH are both involved in the DNA cleavage. Comparative studies with the two sensitizers are also discussed.