The presence of serum antibodies reacting with nucleic acids, nuclear proteins, or nucleoprotein complexes is one of the hallmarks of systemic lupus erythema-

The presence of serum antibodies reacting with nucleic acids, nuclear proteins, or nucleoprotein complexes is one of the hallmarks of systemic lupus erythematosus (SLE) j and related disorders (for a recent review, see reference 1). For reasons that are not well understood, antinuclear antibodies in these disorders often appear to react preferentially with nucleoprotein complexes (1-9). Serum autoantibodies to RNA-protein complexes, including the Sm and RNP (2-5), Ro (6, 7), and La (8, 9) antigens have provided important information regarding the molecular biology of ribonucleoproteins and the immune responses to these antigens in autoimmune diseases. Fewer instances of autoantibodies to DNAprotein complexes have been described (10-15). The best-characterized example is that of antihistone antibodies (14-15). Previous studies from our laboratory (16 and Reeves, Conner, and Kunkel, unpublished observations) have identified a number of high molecular weight nuclear protein antigens, recognized by small subsets of human autoimmune sera, that are distinct from the Sin, RNP, Ro, and La antigens. It would be of interest to know whether or not these poorly characterized autoantigens are also ribonucleoprotein or deoxyribonucleoprotein complexes. The usefulness of autoimmune sera in characterizing these new antigens is limited by the small number of available sera, and by the presence of more than one autoantibody specificity in most autoimmune sera. Monoclonal antibodies (mAb) reacting with the antigens would eliminate many of the problems encountered in using autoimmune sera. Although nuclear proteins are characteristically poor immunogens, mAb against Sm (17), DNA (18), and RNA (19) have previously been obtained from autoimmune mice. In the present studies, antinuclear mAb against a DNAbinding protein complex have been obtained by immunization of BALB/c mice. The deoxyribonucleoprotein complex recognized by the mAb is also recognized