The effect of actarit (MS-932, CAS 18699-02-0), an antirheumatic drug, on type II collagen (CII)-induced arthritis in DBA/1J mice was studied. Mice were immunized twice with bovine CII, actarit being given orally once a day for 35 days after the 1st immunization. Clinical assessment showed that actarit had no effect on the incidence or day of onset of arthritis but that it lowered the arthritis score dose-dependently. Radiography showed that actarit reduced new bone formation in the limbs, and a histopathologic examination showed that it reduced synovitis, erosion of cartilage and bone destruction. Actarit suppressed the delayed-type mouse ear skin reaction to CII but had no effect on the level of serum anti-CII antibodies. These results suggest that actarit inhibits the development of CII-induced arthritis in mice by suppressing delayed-type hypersensitivity to CII.