IFN beta-1b reduces the frequency of major multiple sclerosis attacks by 50 percent. Serial MRI scanning over the course of the clinical trial that led to approval of the agent revealed a significant lessening both in disease activity and in accumulating burden of disease in IFN beta-1b-treated patients compared to placebo-treated controls. The mechanism by which IFN beta-1b exerts its beneficial effect in multiple sclerosis is unknown. T suppressor cell function fails during MS attacks and is persistently subnormal in multiple sclerosis patients with progressive disease. IFN beta-1b partially restores suppressor function in multiple sclerosis patients. IFN beta-1b also inhibits release of lymphotoxin, tumor necrosis factor, and interferon gamma, at least in vitro. All three cytokines are toxic to oligodendrocytes. In contrast; production of transforming growth factor beta-1 (TGF beta 1) is increased by IFN beta-1b. TGF beta 1 is an immunosuppressive cytokine. All of the above listed actions of IFN beta-1b could contribute to its beneficial effect. Perhaps all do.