Acetylcholine receptor now can be visualized in the muscle sarcolemmal membrane with the use of an immunoperoxidase staining of alpha-bungarotoxin (α BT), a substance that binds specifically to the acetylcholine receptor. This technique has allowed new observations in various neuromuscular diseases in which the acetylcholine receptor is affected. In normal muscle, the acetylcholine receptor is confined to the neuromuscular junction. In both experimental denervation and human denervating illnesses, the acetylcholine receptor becomes present diffusely along the muscle sarcolemmal membrane in denervated fibers. In myasthenia gravis, a circulating factor that blocks α BT binding to the acetylcholine receptor of either normal neuromuscular junctions or denervated sarcolemmal membranes is present in 68 percent of serums tested.