Multiple sclerosis is an inflammatory demyelinating disorder of the central nervous system. Immunological evidence from patients with multiple sclerosis and experimental models suggests that the cytokine interleukin-12 (IL-12) plays an important role in the pathogenesis of inflammatory demyelination. In experimental autoimmune encephalomyelitis, an animal model of multiple sclerosis, antibodies that block IL-12 can prevent relapses of the disease. Anti−IL-12 antibodies present a novel approach for preventing relapses in multiple sclerosis.