Summary: A three-layer indirect immunofluorescence technique was used to study the classes of immunoglobulins responsible for AMF and ANF in sera from patients with myasthenia gravis. AMF was found in IgG but not in IgA or IgM. In contrast ANF was detected in all three major classes. In individual sera this reactivity occurred in one, two or three immunoglobulin classes. Evidence is presented that ANF and AMF, although frequently found in the same serum, behave as separate serologic entities.