The ddY mice are known to develop spontaneous glomerulonephritis resembling human IgA nephritis after 40 weeks of age. A sharp rise of circulating polyclonal IgG and IgA is also observed at this stage. Since these overproduced immunoglobulins seem to be related to the development of murine glomerulopathy, antigen-antibody interactions between renal tissue proteins and serum immunoglobulins were analyzed by Western blotting in ddY mice before and after 40 weeks of age. Serum IgG at 50 weeks reacted with an 18-kDa renal tissue protein which was identified as histone H3, as well as with histone H1. Renal histones were extracted along with IgG from the murine kidney at 50 weeks in a high salt soluble fraction. Serial studies of anti-histone antibodies by enzyme-linked immunosorbent assay showed that IgG class antibodies markedly increased after 40 weeks of age. IgA class antibodies mildly increased after 56 weeks of age. Anti-DNA antibodies were not detected. These results demonstrate that ddY mice also develop mainly IgG class and partly IgA class anti-histone autoantibody after 40 weeks of age, and that histone-anti-histone complexes may contribute to the development of murine glomerulopathy. Although anti-histone antibodies have been reported in lupus mice, ddY mice differ from these mice in that no anti-DNA antibodies develop.