Copolymer 1 is a synthetic amino acid copolymer, effective in suppression of experimental allergic encephalomyelitis (EAE) induced in a variety of species. Copolymer 1 can suppress both acute and chronic relapsing EAE induced by either whole brain homogenate or the purified encephalitogens myelin basic protein (MBP) and proteolipid protein (PLP). Thus, the suppressive effect of copolymer 1 in EAE is a general phenomenon and is not restricted to a certain species, the disease type, or the encephalitogen used for EAE induction. The suppressive activity of copolymer 1 is, however, limited to EAE, and copolymer 1 has no nonspecific immunological activity. On the other hand, a marked degree of immunological cross-reactivity in both the cellular and humoral immune responses was demonstrated between MBP and copolymer 1. This cross-reactivity may be the underlying mechanism for the specific suppressive effect of copolymer 1 in EAE. In vivo and in vitro studies using both murine and human cell cultures suggest that the mechanism for copolymer 1 activity in EAE and multiple sclerosis involves, as an initial step, the binding of copolymer 1 to the major histocompatibility complex class II molecules on antigen-presenting cells. Following this step, two pathways may be activated: (1) induction of antigen-specific suppressor T cells by determinants shared between MBP and copolymer 1, or (2) competition with MBP and other myelin-associated antigens, PLP and myelin oligodendrocyte glycoprotein, for the activation of effector T cells. These two mechanisms can act either separately or in concert to interfere in the autoimmune processes that lead to the neurological damage in EAE and multiple sclerosis.