Safety and Efficacy of Natalizumab in Combination With Avonex in the Treatment of Multiple Sclerosis

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