The synthetic random amino acid copolymer Copolymer 1 (Cop 1, Copaxone, glatiramer acetate) suppresses experimental autoimmune encephalomyelitis, slows the progression of disability, and reduces relapse rate in multiple sclerosis (MS). Cop 1 binds to various class II major histocompatibility complex (MHC) molecules and inhibits the T cell responses to several myelin antigens. In this study we attempted to find out whether, in addition to MHC blocking, Cop 1, which is immunologically cross-reactive with myelin basic protein (MBP), inhibits the response to this autoantigen by T cell receptor (TCR) antagonism. Two experimental systems, “prepulse assay” and “split APC assay,” were used to discriminate between competition for MHC molecules and TCR antagonism. The results in both systems using T cell lines/clones from mouse and human origin indicated that Cop 1 is a TCR antagonist of the 82–100 epitope of MBP. In contrast to the broad specificity of the MHC blocking induced by Cop 1, its TCR antagonistic activity was restricted to the 82–100 determinant of MBP and could not be demonstrated for proteolipid protein peptide or even for other MBP epitopes. Yet, it was shown for all the MBP 82–100-specific T cell lines/clones tested that were derived from mice as well as from an MS patient. The ability of Cop 1 to act as altered peptide and induce TCR antagonistic effect on the MBP p82–100 immunodominant determinant response elucidates further the mechanism of Cop 1 therapeutic activity in experimental autoimmune encephalomyelitis and MS.