Ciclosporin A (CYA) is a newly developed immunosuppressant and has a sparing effect on suppressor cell regulatory mechanism. However, accumulation of experimental animal studies has not been enough for the administration of CYA to human autoimmune diseases such as lupus nephritis. In this study, the histopathological aspects of the kidney in (NZB x NZW)F1 mice after administration of CYA were analysed. A dose of 25 mg/kg/day of CYA was injected intraperitoneally so that serum level of CYA was controlled from 250 to 300 ng/ml with this dosage. Typical histopathological changes of lupus nephritis including wire-loop lesions were observed in the kidney of the non-treated control group. In contrast, only few lesions were demonstrated in the CYA-treated group. These results suggest that CYA prevented the renal deterioration in lupus mice.