Interferon-β1b (IFNβ1b) was approved recently for the prophylactic treatment of relapsing–remitting multiple sclerosis (MS). In a controlled trial IFNβ1b at a dose of 8 million units s.c. every other day reduced the frequency of MS attacks by 35% and of major attacks by 50%. Accumulating disease burden as assessed by serial magnetic resonance imaging (MRI) scanning was reduced significantly by IFNβ1b treatment, as was the frequency of MRI scans showing ongoing disease activity. IFNβ1b is well tolerated and appears safe, though flu-like symptoms are frequently encountered upon induction and local erythema at injection sites is usual. The mechanism(s) by which IFNβ1b lessens MS attack frequency is not known. Possible mechanisms include inhibition of viral replication and hence the severity of viral infections, a known trigger for MS attacks; inhibition of immune cell proliferation; altered cell trafficking into and out of the lymphoid organs and into the central nervous system; interference with the antigen-presenting capacity of macrophages; reduced synthesis and release of the stimulatory cytokine IFNγ and of the toxic cytokines lymphotoxin and tumor necrosis factor; and augmented CD8 suppressor cell function and increased synthesis and release of the suppressive cytokines transforming growth factorβ1 and interleukin-10.