We investigated the activity of ion channels in epithelial cells from human thymus glands and thymomas kept in short-term cell culture by clamping the membrane potential of the cells at -85 mV and determining the membrane current flowing on application of acetylcholine, glycine, or gamma-aminobutyric acid. In concentrations of up to 10(-3) M, none of the neurotransmitters induced any detectable current. This suggests (1) that there are no acetylcholine receptors (AChRs) or other products of the AChR gene family having ion-channel properties in the membranes of these epithelial cells, and (2) that the alpha-bungarotoxin-binding protein of thymus and thymoma has no AChR-like ion-channel property. These results support the hypothesis that the cross-reacting structures that elicit the anti-AChR autoimmune response in thymoma-associated myasthenia gravis are antigens having only limited homology with the AChR. Myasthenia gravis not associated with thymoma might have a different pathogenesis.