Using rocket immunoelectrophoresis and indirect haemagglutination tests, antibodies to nicotinic acetylcholine receptor (n-AChR) from Torpedo marmorata were detected in sera from rabbits with experimental myasthenia but not in sera from patients with myasthenia gravis or in rabbit antisera to a partly purified skeletal muscle antigen. Antibodies to this antigen were demonstrated in sera from patients with myasthenia gravis but not in sera from rabbits with experimental myasthenia. The results indicate that there is no immunological cross-reaction between the muscle antigen and n-AChR from Torpedo marmorata, and little or no cross-reactivity between these n-AChR antibodies and possible muscle n-AChR antibodies in myasthenia gravis patients.