The findings in two cases of congenital myasthenia investigated by intercostal muscle biopsy are presented. The first case, a 16-year-old boy, showed reduced miniature endplate potential amplitude and normal 125I-alpha-bungarotoxin binding to postsynaptic acetylcholine receptors. Muscle biopsy and endplate ultrastructure were normal. Tubocurarine affinity, ion channel properties, and passive membrane properties were normal. Limited data showed reduced effectiveness of applied acetylcholine in opening ion channels. The second case was an 18-year-old girl with consanguineous parents. Type 2 muscle fiber atrophy was seen in both limb and intercostal muscle. Intercostal endplates were elongated, although ultrastructure was normal. Negligible postsynaptic alpha-bungarotoxin binding suggested an abnormality of the acetylcholine receptor macromolecule.