The purpose of this study is to determine if natalizumab in combination with Glatiramer Acetate (GA) is safe and effective in delaying progression of individuals diagnosed with relapsing-remitting Multiple Sclerosis (MS).

Inclusion Criteria:

• Diagnosis of MS as defined by McDonald et al., criteria # 1-4
• Between the ages of 18 and 55, inclusive
• Baseline EDSS score between 0.0 and 5.0, inclusive
• Have been treated with GA for at least the 12 months prior to randomization

Exclusion Criteria:

• Primary progressive, secondary progressive or progressive relapsing MS
• MS relapse has occurred within the 50 days prior to randomization
• A clinically significant infectious illness
• History of, or abnormal lab result indicative of significant disease that would preclude the administration of a recombinant humanized antibody immunomodulating agent or GA for 20 weeks.