RECENT STUDIES HAVE SHOWN THAT IT IS POSSIBLE TO ELICIT IN EXPERIMENTAL ANIMALS AN ANTIBODY RESPONSE TO MOLECULES OF NUCLEIC ACIDS IRRADIATED WITH ULTRAVIOLET LIGHT. THE ANTIBODIES OBTAINED SHOW AN ELECTIVE SPECIFICITY FOR THE PHOTOPRODUCTS OF THE THYMIC BASE OF DEOXYRIBONUCLEIC ACID. THE USE OF THESE ANTIBODIES IN INDIRECT IMMUNOFLUORESCENCE METHODS HAS BEEN A VALUABLE AID IN STUDYING THE EFFECTS OF ULTRAVIOLET IRRADIATION ON ANIMALS EXPOSED IN VIVO TO SUCH RADIANT ENERGIES. IT WAS ASCERTAINED THAT THE NUCLEI OF THE CUTANEOUS CELLS OF ANIMALS EXPOSED TO ULTRAVIOLET LIGHT UNDERGO A TEMPORARY DENATURATION, WHICH CAN BE REVEALED WITH THE USE OF ANTISERA SPECIFIC FOR UV-DNA.