The binding of Graves' disease autoantibodies and xenogeneic antibodies to the human TSH receptor (TSH-R) has been studied using receptor preparations generated in an in vitro transcription and translation reaction. The complementary DNAs encoding for the full-length (764 amino acids) and the extracellular region of TSH-R (amino acids 20-414, lacking the signal sequence) were used to generate the translated receptor antigen. Stable [35S]methionine-labeled nascent protein for full-length and extracellular regions of TSH-R of approximate size 87 and 50 kDa, respectively, together with other smaller proteins were generated. The 87- and 50-kDa translated receptor proteins react by immunoprecipitation analysis with monoclonal antibodies, polyclonal antisera, and unfractionated Graves' disease serum containing autoantibody to TSH-R. The translated products of the extracellular TSH-R were examined in detail. Using three well characterized murine monoclonal antibodies whose epitopes encompass the amino-, central, and carboxyl-terminals of the extracellular region of the receptor led to immunochemical identification of the smaller translated products to derive from internal methionine start sites of TSH-R. These smaller, N-terminal-truncated translated proteins were also recognized by polyclonal antisera generated against recombinant TSH-R, thus allowing epitope mapping of some antibodies. A large proportion of Graves' disease autoantibodies (> 70%) bind to the translated extracellular region of TSH-R. This indicates that the majority of pathogenic anti-TSH-R autoantibody binds the nascent translated extracellular region of TSH-R, which is not influenced by the lack of glycosylation of the receptor.