Copolymer 1 (Cop-1) is a synthetic amino acid copolymer effective in suppression of experimental allergic encephalomyelitis (EAE). The suppressive effect of Cop-1 in EAE is a general phenomenon and is not restricted to a particular species, disease type or encephalitogen used for EAE induction. In phase III clinical trials Cop-1 was found to slow progression of disability and reduce the relapse rate in exacerbating-remitting multiple sclerosis (MS). In vivo and in vitro studies suggest that the mechanism for Cop-1 activity in EAE and MS involves the binding of Cop-1 to major histocompatibility complex class II molecules as an initial step. This binding results both in competition with myelin antigens for T-cell activation and in induction of specific suppressor cells of the Th2 type. As an antigen-specific intervention, Cop-1 has the advantage of reduced probability of long-term damage to the immune system.