Aims: Mesangioproliferative glomerulonephritis is a common kidney disease and at present, there is no effective treatment. Our previous studies have demonstrated that Sairei-to can significantly prevent progression of experimental glomerulonephritis in rats. Although we have reported that the active component of Sairei-to in treatment of glomerulonephritis was Saikosaponin-d (Ssd), mechanism of Ssd in prevention of mesangioproliferative glomerulonephritis progression is still unknown. Therefore, current study examines the effects of Ssd on progression of mesangioproliferative glomerulonephritis induced by anti-Thy1 monoclonal antibody 1-22-3 (mAb 1-22-3) in uninephrectomized rats. Methods: Eighteen female Wistar rats first received uninephrectomy and mAb 1-22-3 injection and were then divided into 3 groups: treated daily with phosphate-buffered saline (PBS), 0.6 or 1.8 mg/kg of Ssd. Urinary protein concentration and systolic blood pressure were evaluated and the kidneys were collected and subjected to histological and immunohistological evaluation. The mRNA and protein of the kidneys were extracted and subjected to reverse transcriptase polymerase chain reaction and Western blot analysis, respectively. Results: Ssd reduced the amount of urinary protein and systolic blood pressure. Ssd administration also decreased extracellular matrix expansion, crescentic formation as well as infiltration of macrophages and CD8+ T lymphocytes. Moreover, Ssd significantly reduced expression of transforming growth factor beta 1 (TGF-beta 1) and type I collagen in the kidneys. Conclusion: Ssd inhibits the progression of mesangioproliferative glomerulonephritis through reduction of the expression of TGF-beta 1 and the infiltration of macrophages and CD8+ T lymphocytes. Copyright (C) 2005 S. Karger AG, Basel.