The purpose of this study was to establish the efficacy and safety of two different doses of alemtuzumab (Lemtrada™) as a treatment for relapsing-remitting multiple sclerosis (MS), in comparison with subcutaneous interferon beta-1a (Rebif®). The study enrolled participants who had received an adequate trial of disease-modifying therapies but experienced at least 1 relapse during prior treatment, and who met a minimum severity of disease as measured by magnetic resonance imaging (MRI). Participants had monthly laboratory tests and comprehensive testing every 3 months.