The primary objective is to evaluate the safety and tolerability of natalizumab (BG00002, Tysabri®) in the study population (Russian participants with relapsing remitting multiple sclerosis). The secondary objectives are to look at evaluation of severity of relapse, hospitalization and steroid use requirement; Expanded Disability Status Scale (EDSS), functional tests, quality of life self-assessment questionnaires including the short form health survey self-assessment questionnaire (SF-36) and multiple sclerosis impact scale 29 (MSIS-29), evidence of MRI disease activity, participants free of disease activity (clinical activity and/MRI activity) and anti JC Virus (JCV) antibody evaluation.

Key Inclusion Criteria:

• Must be natalizumab naïve.
• Must have a documented diagnosis of a relapsing remitting form of MS as defined by the revised McDonald Committee criteria (Polman et al., 2011)
• Must have had at least 1 relapse in the previous year:
• Must be stable in disability for at least 30 days prior to enrollment to the study
• Must be stable in symptomatic management of the disease, specifically spasticity, depression and fatigue for at least 30 days prior to enrollment to the study.
• Must be considered by the Investigator to be free of signs and symptoms suggestive of Progressive multifocal leukoencephalopathy (PML) based on medical history, physical examination, or laboratory testing.
• Must be willing to discontinue and remain free from concomitant immunosuppressive or immunomodulatory treatment (including IFN-beta and Glatiramer Acetate) while being treated with natalizumab during the study.

Key Exclusion Criteria:

Medical History:

• Onset of a relapse within 50 days prior to first infusion.
• Considered by the Investigator to be immunocompromised, based on medical history, physical examination, or laboratory testing or due to prior immunosuppressive treatment
• History of, or available abnormal laboratory results indicative of, any significiant viral, cardiac, endocrinologic, hematologic, hepatic, immunologic, metabolic, urologic, pulmonary, gastrointestinal, dermatologic, psychiatric (including major depression), renal, and/or other major disease that would preclude the administration of a recombinant humanized antibody immunomodulating agent. The Investigator must re-review the subject's medical fitness for participation and consider any diseases that would preclude treatment.
• History of malignancy (subjects with basal cell carcinoma that has been completely excised prior to study entry remain eligible)
• Known history of human immunodeficiency virus infection or hematological malignancy
• History of organ transplantation (including anti-rejection therapy)
• A clinically significant infectious illness (e.g. abscess, pneumonia, septicemia) within 30 days prior to the Screening Visit.

Treatment History:

- Treatment with any kind of immunosuppressant medications (e.g., mitoxantrone, cyclophosphamide, cyclosporine, azathioprine, methotrexate, fingolimod, cladribine) within 6 months prior to Screening

Miscellaneous:

• Female subjects who are not postmenopausal for at least 1 year, surgically sterile (does not include tubal ligation), or unwilling to practice effective contraception (as defined by the Investigator) during the study
• Women who are breastfeeding, pregnant, or planning to become pregnant while on study
• Other unspecified reasons that, in the opinion of the Investigator and/or Biogen Idec, make the subject unsuitable for enrollment into this study.

NOTE: Other protocol defined Inclusion/Exclusion criteria may apply