OBJECTIVE To assess whether nucleosome-restricted antibodies, i.e., antibodies that react with the whole nucleosome particle but not with its individual components (double-stranded DNA [dsDNA] and histones), are present in the sera of patients with systemic lupus erythematosus (SLE). METHODS Antibodies were detected by enzyme-linked immunosorbent assay using purified nucleosomes, dsDNA, or histones. These tests were applied to the sera of 40 patients with SLE. Protein G-purified IgGs of representative sera were sequentially adsorbed on dsDNA- and histone-conjugated solid-phase supports and further assayed for their nucleosome, dsDNA, and histone reactivities. RESULTS Of the 40 sera tested, 16 displayed anti-dsDNA and/or antihistone antibody activity, which was always associated with significant antinucleosome reactivity. In addition, 3 sera showed antinucleosome activity that was not associated with concomitant anti-dsDNA or antihistone activity. The presence of true nucleosome-restricted antibodies was demonstrated, after solid-phase adsorption, in representative SLE sera that showed anti-dsDNA or antihistone antibody activity, and also in sera that did not show these activities. CONCLUSIONS Our results provide evidence for the presence of nucleosome-restricted antibodies in patients with lupus. These nucleosome-restricted antibodies, along with anti-dsDNA and antihistone antibodies, appear to belong to a broad set of antinuclear antibodies, the antinucleosome family.