A soluble preparation of nucleoprotein (sNP), a complex of native deoxyribonucleic acid (DNA) and histones, was isolated from calf thymus nuclei and labeled with [(125)I]iodide. Isotope-labeled antigen ([(125)I]sNP) was used in a primary binding radioimmunoassay method to detect antibodies to both sNP and native DNA. Sera with antibody to native DNA reacted with the DNA moiety of sNP and bound [(125)I] sNP, but this binding was completely inhibited by addition of unlabeled native DNA. Antibody to sNP which reacted with DNA-histone complex was not inhibited in the radioimmunoassay by addition of unlabeled DNA. Thus, antibodies to sNP and native DNA could be detected and differentiated by use of a single isotopically labeled antigen. In systemic lupus erythematosus (SLE), sera with binding to [(125)I]sNP was present in 21/36 (58%) patients. The majority (18/21) had antibodies to sNP and native DNA present simultaneously, one had antibody only to sNP and two had antibody only to DNA. In contrast, patients with other connective tissue diseases rarely showed binding to [(125)I]sNP. Serial studies on SLE patients showed that high serum binding to [(125)I]sNP paralleled renal disease activity as reflected by the degree of proteinuria. A fall in binding was observed with subsidence of renal disease and reappearance of increased binding coincided with exacerbation. In these patients, antibodies to sNP and DNA appeared or disappeared pari passu suggesting that in addition to the previously demonstrated role of antibody to native DNA, antibody to sNP might also be implicated in the pathogenesis of immunologically-mediated tissue lesions such as SLE nephritis.