Monoclonal anti-DNA antibodies from 13 cloned hybridoma cell lines have been used to analyze the clonal heterogeneity of autoimmune anti-DNA antibodies. The hybridomas were obtained by fusing spleen cells from a single, autoimmune (NZB x NZW)F1 mouse. With the use of the criteria of isotype, isoelectric focusing patterns, fine specificity for various nucleic acids, and idiotype, the monoclonal anti-DNA antibodies were found to be heterogeneous in that 12 different clonotypes were identified among the 13 monoclonal antibodies studied. However, eight out of the 13 monoclonal antibodies have similar antigen specificity and share a common idiotype. This suggests that although autoimmune anti-DNA antibodies within a single (NZB x NZW)F1 mouse are the products of a large number of different antibody-producing clones, many of the clones produce antibodies with similar antigen-binding regions. Furthermore, in the majority of (NZB x NZW)F1 mouse sera the appearance of detectable levels of the common idiotype coincides with the appearance of anti-DNA autoantibody.