Antibodies to the acetylcholine receptor are present in the serum of myasthenic patients but one does not know if, in vivo and in situ, they can penetrate the intact neuromuscular junction and block directly the receptor. The present experiments demonstrate that molecules the size of antibodies can reach acetylcholine receptor in situ. The mouse diaphragm with its intact neuromuscular junction was used as a source of acetylcholine receptor. The receptor was revealed either directly by iodinated alpha-bungarotoxin covalently coupled to IgG or indirectly, once labelled with cobra toxin, by iodinated anti-cobra toxin antibodies.