Two new monoclonal antibodies, one a mouse IgM and the other a human IgM that reacted with guanosine, were compared to human serum antibodies from patients with systemic lupus erythematosus (SLE). The human monoclonal antibody was polyspecific in its binding to the nucleoside bases, whereas the mouse monoclonal antibody was relatively specific for guanosine when compared by using an enzyme-linked immunosorbent assay (ELISA). Neither antibody bound polyguanylic acid or denatured single-stranded (ss) DNA, however. Serum IgG antibodies from seven patients with SLE cross-reacted with the mouse monoclonal antibody and showed considerable specificity for guanosine. In contrast, the human serum IgG antiguanosine antibodies also bound ssDNA but not dsDNA or polyguanylic acid. Serum IgG antibodies to guanosine measured by ELISA from the seven SLE patients had a decreased response when compared to the total serum IgG response to ssDNA, and most of the antibodies that bound guanosine also bound ssDNA. These studies provide new evidence that there are specific IgG antibodies to guanosine in SLE sera that are a small fraction of the antibodies to ssDNA. Further efforts to define the role of these guanosine antibodies in SLE may provide a better understanding of the basic mechanisms responsible for the development of SLE in man.