Studies of the binding of 125I-labeled α-bungarotoxin to myasthenic motor end-plates have been interpreted as showing a decrease in the number of acetylcholine (ACh) receptors at these end-plates. Equilibrium binding studies of 125I-tagged α-bungarotoxin to detergent-extracted ACh receptors from normal and myasthenic intercostal muscle were carried out to determine whether the reduced toxin binding previously reported could be due to a reduced affinity of myasthenic receptors for α-bungarotoxin rather than to a decreased number of receptors. Our results show increased rather than decreased affinity of myasthenic receptors for α-bungarotoxin and also suggest that the number of ACh receptors is indeed reduced. The presence of a change in binding affinity, in addition to the reduced number of ACh receptors, suggests the presence of membrane changes that may contribute to the pathogenesis of myasthenia gravis.