Summary BDB indirect hemagglutination was determined to be an effective measure of antibody to thermally denatured DNA. It was found equally as effective as bentonite agglutination in detecting anti-DNA in SLE sera. Antibodies to thermally denatured DNA were detected in the sera of experimentally-infected tuberculous rabbits. The antibodies appeared 4-6 weeks post-infection, reached a maximum titer at approximately 8 weeks, and declined thereafter. Similar anti-DNA antibodies were also detected in the sera from patients with tuberculosis or histoplasmosis. Dr. Roy L. Donnerberg, Benjamin Franklin Tuberculosis Hospital, Columbus, Ohio, generously provided the human tuberculous sera employed in this investigation. Human histoplasmosis sera were obtained through the courtesy of Dr. J. A. Schmitt, Department of Botany, Ohio State University; Dr. Tom D. Y. Chin, Kansas City Communicable Disease Center; Miss Jo Ann Sparks, Infections Disease Center, University Hospital, Columbus, Ohio. Normal human sera were obtained from the University Hospital Blood Bank, Columbus, Ohio. Only those serum samples from acceptable blood donors who demonstrated no overt signs of disease were employed.