Serum from a patient with systemic lupus erythematosus (SLE) containing antibodies to native DNA was compared with antiserum from a rabbit immunized with heat-denatured DNA. DNA was fractionated on methylated albumin kieselguhr into native and denatured forms, which were used as antigens in immunologic tests. By immunodiffusion, complement fixation and immunofluorescence, rabbit antiserum reacted only with denatured DNA and was completely unreactive with native DNA. SLE serum reacted strongly with native DNA in all tests and weakly with denatured DNA in complement-fixation and immunofluorescence tests. These studies demonstrated the importance of employing purified preparations of native or denatured DNA to differentiate between DNA antibodies of different specificities. They also showed that DNA antibodies experimentally induced by immunization with heat-denatured DNA did not cross-react with purified native DNA. An immunofluorescence test was described which showed that antibodies to native DNA stained DNA in the form of strands, and antibodies to denatured DNA stained DNA in the form of aggregated granules.