The terminal and lytic complement component (C9) was localized at the motor end-plate in acquired autoimmune myasthenia gravis (MG) by the immunoperoxidase method, with adequate preservation of fine structure and negligible background staining. C9 was localized on short segments of the postsynaptic membrane, on degenerated fragments of the junctional folds shed into the synaptic space, and on disintegrating junctional folds. An inverse relationship was noted between the structural integrity of the junctional folds and the abundance of C9 at a given endplate region. Destruction of junctional folds by complement may induce relocation of the nerve terminal and increased spatial separation of end-plate regions on the muscle fiber. Destruction of junctional folds by the complement membrane attack complex is a cause of the acetylcholine receptor deficiency at the MG end-plate, but antibody-dependent modulation of the receptor may also contribute to deficiency of the receptor. In certain disorders other than autoimmune MG, pathological mechanisms other than complement-mediated lysis may affect the structural integrity of the postsynaptic region.