Anti-histone antibodies have been demonstrated in the sera of patients with both idiopathic and drug-induced lupus. We measured anti-histone antibodies in female NZB/NZW (F1) mice, which are considered to be a model of human SLE. Using a sensitive and quantitative enzyme-linked immunosorbent assay (ELISA), we detected minimal serum antibody activity in NZB/NZW mice younger than 4 mo of age and in nonautoimmune mice at all ages tested. Serum anti-histone antibodies progressively increased in NZB/NZW mice from 4 to 8 mo of age and showed an age-related maturation from IgM to IgG. The predominant antibody activity in the older mice was to the individual histones H2B and H3, and the pattern of reactivity to the histone proteins was similar to that seen in human SLE. We also studied the spontaneous in vitro production of anti-histone antibodies using spleen cells from NZB/NZW mice of different ages. Culture supernatants were analyzed for antibody activity by an ELISA with total histones as the antigen. Spleen cells from older NZB/NZW mice, with elevated serum levels, produced 10-fold higher levels of antibody activity compared to age-matched nonautoimmune mice. Antibody production was maximal at 4 days of culture and was inhibited by the addition of puromycin to the culture. Surprisingly, spleen cells from 1 to 3-mo-old NZB/NZW mice, with normal serum levels, also demonstrated significantly elevated production. The antibodies produced by these young mice were mostly IgM, whereas spleen cells from older mice produced mostly IgG anti-histone antibodies. The present results provide the basis for using the anti-histone antibody system to study further the immune abnormalities that allow for autoantibody production.