Competition between peptides for binding to class II major histocompatibility complex (MHC) molecules has been demonstrated both in vitro and in vivo. Peptide competition may provide a way to interfere with T cell activation in the treatment of autoimmune diseases. It may be possible to provide a substitute 'blocking' peptide to compete for presentation of an autoantigenic peptide to T cells. The approach described is a general one, which may be applicable to a number of T cell mediated MHC-linked autoimmune diseases, and to other undesirable immune responses. So far, peptide competitors have only been successfully used in the prevention of experimental autoimmune encephalomyelitis (EAE). Whether or not this approach will work in treating spontaneous disease models remains to be tested, although work in other test systems is encouraging.