MRL/Mp(-)+/+ mice produce antinuclear antibodies and develop a spontaneous autoimmune syndrome with lupus-like nephritis. We obtained a panel of seven histone-reactive IgG mAb from a single MRL/Mp(-)+/+ mouse. These antibodies do not react significantly with DNA or individual histones, but bind strongly to the histone H2A-H2B dimer and even more strongly to the H2A-H2B-DNA complex. These antibodies also bind to whole nuclei when tested by immunofluorescence, indicating that they recognize an epitope accessible in chromatin. The V region sequences of these antibodies have been determined. The H chain third complementarity-determining regions of these antibodies are similar to those found in anti-DNA antibodies even though the antibodies in our panel do not react with DNA in the absence of histones, suggesting that DNA is part of the subnucleosome epitope. Several of these antibodies are clonally related, supporting the hypothesis that the activation of these clones is Ag-driven. Analysis of the sequences of these antibodies indicates that they derive from autoreactive B cells that were clonally expanded and whose V region genes have undergone numerous somatic mutations.