The effects of 1 alpha-hydroxyvitamin D3 and 24,25-dihydroxyvitamin D3 on Type II collagen-induced arthritis in rats, an experimental model of rheumatoid arthritis, were examined. Oral administration of 1 alpha-hydroxyvitamin D3 significantly suppressed the incidence of arthritis and inhibited hind paw swelling. The level of anti-Type II collagen antibodies was decreased in the 1 alpha-hydroxyvitamin D3 treated-group. In contrast, 24,25-dihydroxyvitamin D3, indomethacin, and gold had no effect on either the incidence of arthritis or the antibody levels. These findings demonstrate a beneficial effect of 1 alpha-hydroxyvitamin D3 on Type II collagen-induced arthritis in rats and indicate that it has an antirheumatic effect.