Background: The herbal medicine Sairei-to is efficacious in renal diseases where mesangial proliferation is a key event. We examined whether Sairei-to inhibits proliferation of cultured rat mesangial cells and investigated its mechanism of action. Methods: The effect of Sairei-to on [<sup>3</sup>H]-thymidine incorporation stimulated by serum was assessed. Cell cycle was analyzed by flowcytometry. Extracellular signal-regulated kinase (ERK) activity was determined by immunecomplex kinase assay. Tyrosine phosphorylation of cellular proteins, and phosphorylation of ERK and Raf-1 were analyzed by immunoblot. Cyclic AMP was measured by radioimmunoassay. Results: Incubation of mesangial cells for 18 h with water-soluble but not insoluble fraction of Sairei-to inhibited serum-stimulated [<sup>3</sup>H]-thymidine incorporation. In subsequent experiments, water-soluble fraction, at a dose required for a half-maximal response (2 mg/ml), was used. Sairei-to inhibited S-phase entry stimulated by serum. Serum-induced tyrosine phosphorylation of p44 and p42 ERK was inhibited by Sairei-to, but that of other cellular proteins was not affected. Suppression of serum-stimulated ERK activation by Sairei-to was confirmed by immunecomplex kinase assay. Activation of Raf-1, an upstream activator of ERK, was also attenuated by Sairei-to. Incubation of cells with Sairei-to significantly increased the generation of cAMP. Conclusions: Sairei-to inhibits serum-induced DNA synthesis of rat mesangial cells by suppressing Raf-1/ERK cascade probably via cAMP.