The primary hypothesis of this study is to determine whether the cumulative number of newly active lesions developing over a treatment period of 24 weeks, as seen on MRI scanning performed every 4 weeks, is 50% lower in any of the three Ro 45-2081 treatment groups as compared to placebo. This study aims to determine whether or not Ro 45-2081 has the potential to halt or prevent disease progression in patients with multiple sclerosis by assessing the rate of newly active lesions seen on MRI.