The purpose of this study is to determine if natalizumab in combination with AVONEX 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 Avonex for at least the 12 months prior to randomization
• Have experienced at least one relapse (while on Avonex) within the 12 months prior to randomization.
• Cranial MRI scan demonstrating lesions consistent with MS.
• Have given written informed consent to participate in the study.

Exclusion Criteria:

• Primary progressive, secondary progressive, or progressive relapsing MS.
• MS relapse has occurred within 50 days of randomization
• A clinically significant infectious illness within 30 days prior to randomization
• History of, or abnormal lab result, indicative of significant disease, that in the opinion of the investigator, would preclude the administration of a recombinant humanized antibody immunomodulating agent or Avonex for 116 weeks.
• History of severe allergic or anaphylactic reactions or known drug hypersensitivity.
• Unable to perform the Timed 25-Foot Walk, 9HPT and PASAT 3
• Abnormal blood tests at Screening Visit