OBJECTIVE To develop a monoclonal antibody reagent that would react with nucleosomes but not directly with constituent double stranded DNA (dsDNA) or with histones. METHODS Mice were immunized with highly purified chicken mononucleosomes and hybridomas employed to produce Mab that did not react with dsDNA or histones but still showed reactivity with nucleosomes. RESULTS Murine monoclonal IgG antibody 4H7, generated from a mouse immunized with highly purified chicken erythrocyte nucleosomes, showed no direct ELISA reactivity with either dsDNA or isolated histones or with Sm and RNP antigens or combinations of any of these components. Mab 4H7 did show strong ELISA reactivity for chicken erythrocyte and calf thymus nucleosomes as well as for human leukocyte nucleosomes. The Mab did show strong ELISA reactions with peptides 1-25 of histone H2B and 1-21 of H3, which correspond to sequences known to be located at the surface of nucleosomes. We then measured relative serum levels of 4H7 reactive nucleosome antigen in 140 patients with systemic lupus erythematosus (SLE) in parallel with 50 non-SLE patients with other types of connective tissue disease and 92 healthy subjects. Occasional low levels of serum nucleosomal antigen were seen in 4 of 92 controls, but many patients with SLE (66/140) showed marked elevations of serum nucleosomal antigen. No difference was observed when serum or plasma samples were studied. A marked correlation (R = 0.401, p < 0.0001) was noted when disease activity score (SLEDAI) was plotted against optical density value measured with 4H7 in ELISA. Further, the levels of circulating nucleosomes were raised in SLE patients with very active central nervous system and renal involvement. CONCLUSION Presence of nucleosome related antigen in sera from patients with SLE may provide insight into the sequence of disease related antigenic stimuli in active SLE.