Experimental allergic encephalomyelitis (EAE) is a model for several human diseases including multiple sclerosis and post-vaccinal encephalopathies. EAE is generally thought to be an autoimmune response to the antigen myelin basic protein (MBP). Oddly, MBP can also suppress EAE, and many observations suggest that an independent immune response to so-called "adjuvant" material is also necessary to EAE induction. Thus, EAE may be a result of a pair of interactive immune responses, one against MBP, and one against adjuvant. If so, the adjuvant should, like MBP, suppress EAE. We present data from experiments on strain 13 guinea pigs demonstrating EAE suppression by muramyl dipeptide, an active component of complete Freund's adjuvant. These results are striking because classically adjuvants are defined as immunopotentiators, not immunosuppressants. Our results, therefore, suggest that a revaluation of the role of adjuvants in inducing autoimmune diseases may be necessary.