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