An opportunity to investigate the role of anti-acetylcholine receptor antibody (anti-AcH R-antibody) in neonatal myasthenia gravis was presented when an infant was born to a symptomatic myasthenic mother who elected to breast feed the child. Pyridostigmine bromide determinations in plasma and breast milk were made by quantitative gas liquid chromatography. Anti-AcH R-antibody was assayed by an immunoprecipitation method. Simultaneous maternal blood and milk samples did not suggest concentration of pyridostigmine bromide in milk or significant transfer of medication through demand breast feeding. Weakness was not noted in the neonate in spite of high levels of anti-AcH R-antibody demonstrated in her blood. Presence of a markedly elevated anti-AcH R-antibody in a pregnant patient symptomatic with myasthenia gravis does not necessarily predict a clinically affected offspring, nor does the elevated antibody in the infant, presumably acquired transplacentally, necessarily result in clinical symptomatology in the newborn period.