In a proliferation assay, peripheral blood lymphocytes (PBL) from a relatively small proportion of myasthenia gravis (MG) patients and from controls responded to Torpedo acetylcholine receptor (T-AChR), which shows approximately 75% homology with the human AChR. Over 50% of MG patients responded to recombinant human AChR alpha-subunit (r37-437) however, compared with 9% of controls. A proportion of MG PBL respond to synthetic peptides of the extracellular portion of the human alpha-subunit, but only MG patients (18%) responded to the juxta-membrane sequence p257-269. MG T-cell lines raised against native T-AChR failed to respond to the synthetic peptides. These results underline the need to use human AChR sequences to test relevant T-cell reactivity in MG. T-cell lines raised from three MG patients to human alpha-subunit r37-437 have shown Stimulation Index (SI) values of 3.5-22. Three clones derived from one of these had SI values of 100-500. Preliminary testing of responsiveness in one of these clones showed reactivity to several recombinant polypeptides including r37-437 and r37-181, as in the parent line. The epitope(s) within this latter sequence have not yet been identified, but the experimental approach used here should make it possible to define critical T-cell epitopes in MG, and to determine their functional relevance by investigating the ability of AChR-reactive T-cell clones to provide specific help in anti-AChR antibody production.