Every participant received active treatment; there was no placebo. After Amendment 2, the 24 mg alemtuzumab dose was closed to enrollment so newly enrolled participants were randomly assigned to treatment with either 12 mg alemtuzumab or interferon beta-1a in a 2:1 ratio (that is, 2 given 12 mg alemtuzumab for every 1 given interferon beta-1a). Alemtuzumab was administered in two annual courses, once at the beginning of the study and again 1 year later. Interferon beta-1a was self-injected 3 times per week for 2 years. All participants were required to return to their study site every 3 months for neurologic assessment. In addition, safety-related laboratory tests were performed at least monthly. Participation in this study ended 2 years after the start of treatment for each participant. Additionally, participants who received alemtuzumab might be followed in the CAMMS03409 Extension Study (NCT00930553) for safety and efficacy assessments. Participants who received interferon beta-1a and completed 2 years on study might be eligible to receive alemtuzumab in the Extension Study.