We examined the therapeutic effect of 15-DOS in the two models of acute and chronic relapsing EAE in Lewis rats. In the first model adult rats developed an acute severe EAE and by day 16 all animals died. Lewis rats treated with 15-DOS showed a delayed and reduced onset of clinical symptoms and mortality was prevented. In the second model Lewis rats (aged animals) developed a chronic relapsing EAE with up to three relapses. The second and third episodes were both milder and shorter in duration. All animals treated with 15-DOS survived the delayed first attack and developed no further relapses.