Chronic glomerulonephritis (GN) was induced in N/M mice by daily injections of human serum albumin (HSA). The glomerular lesion was similar to that observed in human membranous GN and was characterized by intense mesangial and capillary loop immunofluorescent staining for HSA, IgG and C3. Electron microscopic examination revealed numerous electron-dense deposits in the mesangium and along the subepithelial side of the glomerular basement membrane, the latter deposits being associated with membranous spikes. Chronically injected mice that had been treated with cyclosporin (CsA) from Day 1 had different patterns of immune complex deposition. Mesangial deposition was apparently unaltered but no subepithelial deposits or spikes were evident. In addition, only two out of 21 HSA-injected mice which began CsA treatment on Day 21 had subepithelial deposits. There was no significant difference in serum levels of HSA-specific IgG between the three groups of mice. CsA treatment would therefore appear to ameliorate the immunopathology of antigen-induced glomerulonephritis in this model without affecting serum antibody levels, and may be of therapeutic value in the treatment of human membranous GN.