Autoimmune Th cells in myasthenia gravis recognize several sequence regions of the human muscle acetylcholine receptor (AChR). Most AChR Th epitopes are presented by HLA class II DR molecules (DR). Four sequence regions of the AChR alpha-subunit form Th epitopes recognized by most myasthenic patients, irrespective of their DR haplotype. In this study we first identified in five myasthenic patients the DR molecule(s) likely to be involved in presentation of T immunodominant AChR sequences. We then investigated the binding to the affinity purified DR molecules thus identified (DR2/w51, DR4/w53, and DR7/w53) and to the DR1 molecule, of a panel of overlapping synthetic peptides screening the human alpha-subunit sequence, previously used to identify AChR Th epitopes in myasthenic patients. The AChR peptides that stimulated anti-AChR autoimmune Th cells all bound the relevant DR molecules. Some AChR peptides never recognized by Th cells of myasthenic patients also bound well to one or more DR molecules. The relative ability to bind to DR molecules of different sequence regions of the AChR, i.e., an autoantigen, agrees well with the results of previous studies on the DR binding of synthetic sequences of exogenous antigens. Some peptide sequences uniquely bound one DR molecule, others bound several DR molecules, and others did not bind any of the DR molecules tested.