The mRNAs encoding heavy and light chains of a hybridoma-derived monoclonal IgM kappa anti-DNA autoantibody from lupus-prone MRL/Mp-lpr/lpr mice (Ighj) have been transcribed into cDNA copies and molecularly cloned, and their complete nucleotide sequences have been determined. The mRNA for the heavy chain variable region, including leader peptide and 5' untranslated region, is transcribed from a heavy chain variable region (VH) gene closely related (and possibly allelic) to VH genes of the C57BL/6 (Ighb) nitrophenyl antibody family. The deduced amino acid sequence corresponding to the light chain variable region of this autoantibody shows extensive similarities with non-autoantibody molecules of the V kappa 1 group, suggesting a common variable gene origin. The joining segments, constant regions, and 3' untranslated regions of both the heavy and light chain mRNAs are nearly identical to corresponding sequences of non-autoantibodies from normal mice. Our findings suggest that this anti-DNA autoantibody originated from the same germline repertoire as antibodies to exogenous antigens.