Six monoclonal IgM from patients with Waldenstrom's macroglobulinemia that react with Klebsiella polysaccharides were tested for their ability to bind to nucleic acid antigens. One of the macroglobulins bound to the polynucleotide poly(G), and one bound to poly(G), poly(I), and single-stranded DNA. The reaction with the polynucleotides was specifically inhibited by the Klebsiella polysaccharide K30. A monoclonal lupus anti-DNA antibody (16/6) was found to react weakly with the Klebsiella polysaccharides K30 and K21. Five of the Waldenstrom macroglobulins shared an idiotypic determinant with the 16/6 anti-DNA antibody. The reaction between the macroglobulins and the antiidiotype serum was specifically inhibited by Klebsiella polysaccharides, an indication that the idiotypic marker was in the antigen-binding site of the macroglobulins. These results indicate the existence of widely dispersed conserved variable region genes that encode idiotypically related immunoglobulins with the capacity to bind to both bacterial polysaccharides and nucleic acids. Such genes can be expressed by patients with either Waldenstrom's macroglobulinemia or systemic lupus erythematosus.