DNA after irradiation with ultraviolet light was immunogenic in rabbits and elicited serum antibodies reacting specifically with UV-irradiated DNA. The serological reactions were demonstrated by immunodiffusion, complement fixation, and immunofluorescence. By immunofluorescence, antisera reacted with cell nuclei of irradiated tissue sections but not with unirradiated tissue. This method was employed to show the presence of UV lesions in tissues of mice exposed to UV light. UV lesions in DNA were present in nuclei of epidermal cells, and in heavily irradiated animals they were also detected in the corium immediately below the epidermis. The method is useful not only for directly demonstrating UV lesions of DNA but also for localizing such lesions in tissues.