Saireitoh is a traditional Chinese medicine that is often given to patients with nephrotic syndrome or glomerulonephritis. Studies have reported that Saireitoh stimulates intrinsic steroid secretion in rats and suppresses the proliferation of fibroblasts in vitro. We examined the effects of Saireitoh on vascular smooth muscle cell proliferation and migration in vitro and experimental atherosclerosis in vivo. Saireitoh rabbit serum obtained from New Zealand White rabbits which were given a diet containing 2% Saireitoh for 3 days significantly inhibited [3H]-thymidine incorporation by smooth muscle cells, which were isolated from thoracic aorta explants of rabbits. The addition of 10% Saireitoh rabbit serum to a culture medium containing smooth muscle cells inhibited DNA synthesis by 50% as compared with a control culture to which 10% normal rabbit serum was added. We also found that the number of smooth muscle cells in the culture containing Saireitoh rabbit serum was decreased. When PDGF was used as a chemoattractant, we demonstrated that Saireitoh rabbit serum slightly inhibits the migration of smooth muscle cells. In in vivo experiments, Saireitoh did not suppress the development of atherosclerosis but tended to reduce the damage. We concluded that although Saireitoh inhibited the proliferation of smooth muscle cells, the effect of prevention on the development of atherosclerosis is weak in the in vivo condition.