Objective. CXCL10 (also known as interferon-␥-inducible 10-kd protein [IP-10]) is a chemokine that potentially plays a role in the immunopathogenesis of rheumatoid arthritis (RA). We undertook this phase II study to evaluate the efficacy and safety of MDX-1100, a fully human, anti-CXCL10 (anti-IP-10) monoclonal antibody, in RA patients whose disease responded inadequately to methotrexate (MTX).Methods. Patients with active RA receiving stable doses of MTX (10-25 mg weekly) were randomized to receive intravenous doses of 10 mg/kg MDX-1100 (n ‫؍‬ 35) or placebo (n ‫؍‬ 35) every other week. The primary end point was the proportion of patients meeting the American College of Rheumatology 20% improvement criteria (achieving an ACR20 response) on day 85, and patients were followed up for safety to day 141.Results. The ACR20 response rate was significantly higher among MDX-1100-treated patients than among placebo-treated patients (54% versus 17%; P ‫؍‬ 0.0024). Statistically significant differences in the ACR20 response rate between treatments were observed starting on day 43 (P < 0.05). The ACR50 and ACR70 response rates on day 85 did not differ between the groups. Overall, 51