Background: Rosmarinic acid is known to be a natural phenolic compound widely distributed in Labiatae herbs such as rosemary, sweet basil, and perilla. In the present study, we evaluated the suppressive effects of rosmarinic acid on mesangioproliferative glomerulonephritis in vivo, which was induced by intravenous injection of rabbit anti-rat thymocyte serum (ATS) to rats. Methods: Rosmarinic acid was orally administered to the rats at a dose of 100 mg/kg/day from the day of ATS injection (day 0) to day 8 when rats were sacrificed. The degree of mesangial cell proliferation and matrix accumulation were evaluated by trichrome staining and by immunostaining for proliferating cell nuclear antigen (PCNA), fibronectin, type IV collagen and fibrin. Superoxide dismutase (SOD)-activity in the homogenate of renal cortex was also evaluated. Results: The number of PCNA-positive cells, staining areas of trichrome, fibronectin, collagen IV and fibrin in the glomerulus were significantly decreased, and SOD-activity of renal cortex homogenate was significantly augmented in rosmarinic acid-treated group. Conclusion: Rosmarinic acid would suppress the proliferation of mesangial cells and glomerular matrix expansion in vivo by its fibrinolytic and anti-oxidative activity.