Anti-idiotypic antibodies were prepared against purified acetylcholine receptor antibodies from two patients with myasthenia gravis. The purified idiotypes did not cross-react. Reaction with idiotypes from other patients were found in 8% and 37%, respectively, which suggests that shared idiotypic specificities occur. The anti-idiotypic IgG fractions had no receptor-like activity and did not bind cholinergic ligands. Receptor antibodies from two mothers and their newborn children with neonatal myasthenia gravis showed marked differences in the reactions with an anti-idiotypic antibody. This suggests that not passive transfer of maternal antibodies but a transient synthesis of a receptor antibody with a different specificity is an important factor in the pathogenesis of neonatal myasthenia gravis.