Autoantibodies against the acetylcholine receptor (AChR) are involved in the neuromuscular dysfunction associated with myasthenia gravis (MG). We determined the nucleotide and the deduced amino acid sequences of the heavy and light chains for one human monoclonal anti-AChR autoantibody, derived from peripheral blood lymphocytes obtained from one MG patient. The heavy and light chain (VH and V lambda) genetic elements used in this autoantibody are respectively or closely related to HHG19 and Humlv117 germline structure. In addition, the expressed JH, J lambda and D segments differ from their described germline structures. This diversity could reflect allelic variation. The large number of differences found in VH, VL and D genetic elements when compared with their most closely related germline structures allowed us to conclude that we have characterized new VH and VL genes and we could not identify univocally somatic mutations. However, the analysed autoantibody, an IgG specific for the alpha chain of the AChR, revealed the presence of N addition segments on both sides of the D region and we may postulate that this antibody was the result of an antigen driven phenomenon.