The purpose of this study is to evaluate the safety and efficacy of three different doses of belimumab (1 mg/kg, 4 mg/kg, and 10 mg/kg), administered in addition to standard therapy, compared to placebo plus standard therapy in patients with RA. All patients were to be dosed on Days 0, 14, and 28, then every 28 days for the remainder of 24 weeks. Patients completing the 24-week period could enter a 24-week open-label extension; belimumab patients received the same dose or were switched to 10 mg/kg at the investigator's discretion and former placebo patients received belimumab 10 mg/kg.