The effects of a formula of traditional Chinese medicine, TBL-1, on collagen-induced arthritis (CIA) were investigated in DBA/1J mice. 2. From 4 weeks after the first immunization with bovine type II collagen (CII), TBL-1 or indomethacin was administered orally for 13 weeks. 3. Clinical scores of CIA were decreased by both TBL-1 and indomethacin intervention compared with the control CII-immunized group. 4. Radiographic scores of phalangeal destruction were markedly improved by TBL-1 intervention (P < 0.001), but indomethacin failed. 5. The suppressive effects of TBL-1, but not indomethacin, were manifested in reduced serum anti-CII antibody titres (P < 0.001). 6. These findings suggest that TBL-1 may play a role in regulating some immune responses in the present arthritis model.