Eighteen neonates were investigated for antibodies directed against acetylcholine receptors. No antibody was detected in 3 cases of congenital myasthenic syndrome, whereas positive results (3.5 to 250 nM) were obtained in 7 cases of transitory neonatal myasthenia and in 7 of 8 asymptomatic infants born to myasthenic mothers. The neonatal antibodies are fully cleared within 1 to 6 months (half-life: 9 days to 2 and a half months). The prognostic value of the maternal and infantile antibody titers is limited: maternal titer at the end of pregnancy, though usually higher in the mothers of myasthenic children, do not predict or preclude the occurrence of transitory myasthenia, and there is no clear correlation between the severity and duration of the myasthenia and the initial titer or the level of the child's antibodies. The assay which measures antibodies against acetylcholine receptors is useful in that it confirms the diagnosis of transitory myasthenia and excludes congenital myasthenic syndromes.