Antibodies, directed against an acetylcholine (ACh) conjugate, were found in the sera of myasthenic patients by using a modified enzyme-linked immunosorbent assay method. Binding of sera from control patients to the conjugate was extremely low. From competition experiments, done with compounds close to ACh, a good antibody specificity for the ACh conjugate was demonstrated in the sera of myasthenic patients. Laboratory tests will be done to help a more precise diagnostic of the disease to be given. These results may also be of value in further understanding of the process of the disease itself.