Interferon beta is a cytokine that modulates immune responsiveness. Treatment with recombinant interferon b, either subcutaneously or intramuscularly, favorably affects the natural history of multiple sclerosis. Attack frequency and attack severity are both reduced and the progression of accumulating disability is slowed. Magnetic resonance image scanning of treated patients reveals a lessening of accumulating permanent disease burden and an up to 80% reduction in newly active lesions, the majority subclinical, within the white matter of the brain. Many newly active lesions resolve spontaneously. The drug is well tolerated and safe. Systemic flu-like symptoms, transient in nature, commonly follow drug injection when treatment begins, but these usually lessen in frequency and severity over the first few weeks on drug. Red skin reactions are usual at sites of subcutaneous injection. Interferon b treatment is only partially effective. There is an unmet need to do better in the therapy of multiple sclerosis.