The biochemical identities of several antigens to which striational antibodies bind were determined by using serum antibodies and monoclonal antibodies from two patients with myasthenia gravis. The monoclonal antibodies were secreted by EBV-transformed B-lymphocyte clones obtained from thymus and thymoma. Serum and monoclonal antibodies reacted with discrete components of the skeletal muscle sarcomere, giving rise to several different patterns of immunofluorescence staining. Immunoblot analyses and enzyme-linked immunosorbent assays revealed three different antibody specificities: myosin, alpha-actinin, and/or actin. Individual monoclonal StrAb reacted with both muscle and nonmuscle isotypes of actin or myosin. It is noteworthy that contractile proteins (1) are associated with acetylcholine receptors (AChR) in plasma membranes, and (2) are biochemically altered in transformed cells. It is therefore conceivable that the release of neoantigenic AChR-associated contractile proteins from thymic epithelial cells undergoing neoplastic transformation may provide the immunogenic stimulus for production of StrAb. More precise definition of StrAb specificities in individual patients with MG and/or thymoma might provide a basis for diagnostic and/or prognostic classification of these diseases. Furthermore, the monoclonal antibodies will be useful in experimentally testing the potential pathogenicity of StrAb.