Myasthenia gravis is an autoimmune disease which may be detected by the presence of serum antibodies against the nicotinic acetylcholine receptor at the neuromuscular junction. Immunoprecipitation assays have been developed to measure these immunoglobulins and calculate titres. These assays require the labelling of the receptor with 125I-α-bungarotoxin which binds irreversibly. However, the standard immunoprecipitation assay may significantly underestimate the titres of some myasthenic patients. We have discovered patients with antibodies specific for the α-bungarotoxin binding site of purified rat muscle receptor. If labelled toxin is already present on the receptor, these antibodies are unable to bind to the protein. This phenomenon may lead to underestimates of the actual antibody titre. To circumvent this problem, we have designed a modified immunoprecipitation assay to evaluate titres.