Two anti-DNA hybridoma autoantibodies ( A52 , D42 ) were prepared by fusing spleen cells from unimmunized NZB/NZW F1 female mice with BALB/c myeloma cells. The monoclonal antibodies were purified to homogeneity and were analyzed for their antigen-binding specificities. The two anti-DNA antibodies bound single-stranded, double-stranded, and supercoiled DNA, with a marked preference for the single-stranded conformation. Competition experiments performed with synthetic polynucleotides, as well as chain reconstitution experiments, indicated that both the sugar-phosphate backbone and the heterocyclic bases of the nucleic acid are essential for antibody recognition. Amino terminal sequence analysis of A52 and two RNA-binding hybridoma proteins revealed that the heavy chains from all three were members of the VHII subgroup and that the A52 light chain was homologous to the VK8 subgroup. The D42 heavy chain was found to be similar to a phosphocholine-binding hybridoma of the VHIII subgroup.