To evaluate the usefulness of a traditional Chinese medicine (Kampo prescription), Keishi-bushi-to (KBT), which is composed of five medicinal plants derived from Kampo prescriptions used to treat rheumatoid arthritis, we investigated the effect of KBT on the development of arthritis induced by type II collagen (CII). Oral administration of KBT at a dose of 500 mg/kg from 7 d before intradermal injection of CII significantly reduced the severity from 7 d after the onset of arthritis. The reduction in body weight resulting from the development of arthritis was not seen in rats treated with KBT. Plasma IgG and IgM anti-CII antibody levels were lower in KBT-treated rats than control rats. In addition, the clearance of IgG anti-CII antibody from circulating blood after intravenous injection was faster in KBT-treated rats than control rats. These results indicate that KBT is effective in suppressing collagen-induced arthritis and its effect is at least partly due to the suppression of humoral and cellular immunity.