The participant will receive weekly intramuscular treatment with AVONEX® (interferon beta 1-a) and bi-monthly high dose intravenous methotrexate with Leucovorin rescue. Entrance into the study will be based on Multiple Sclerosis Functional Composite Score (MSFC) evaluations at -4 months, and - 1 month prior to study, where a decrease in score is noted. The composite score consists of the Timed 25-Foot Walk, Nine-Hole Peg Test (9HPT), and Paced Auditory Serial Addition Test 3 (PASAT 3). The patient must have been on AVONEX® for at least 6 months.