Abstract. When solubilized, radiolabelled membrane preparations from FRTL-5 rat thyroid cells are applied to TSH affinity columns, two separate peaks of protein can be eluted by high salts/high pH and low pH buffers, respectively. Immunoprecipitation with monoclonal antibodies to the TSH receptor shows that both peaks contain proteins related to the TSH receptor. If extracts were from cells grown without TSH, one peak has a ~ 300 K and the other a ~ 70 K protein the 70 K protein can be derived from the purified 300 K protein in vitro. A 50 and 20 K protein can be derived from the 70 K protein. If extracts are from cells grown with TSH, the peaks contain a multiplicity of additional immunoprecipitable bands of ~ 200, 175, 130, 90, 50, 20 K etc. These bands are shown to result from the ability of TSH to increase the synthesis (3–4-fold) and degradation (2–3-fold) of the 300 and 70 K proteins. The 300/70 K protein fractions are reactive with monoclonal autoimmune thyroid stimulating antibodies and contain a specific disialo ganglioside. The ganglioside migrates near GM2, i.e., like a lower order ganglioside, and contains fucose. In translation experiments, the monoclonal antibodies to the TSH receptor identify a single mRNA component which produces a protein of ~ 220 K. This protein is not present in thyroid cells which have no functional TSH receptor and which cannot be surface labelled with monoclonal antibodies to the TSH receptor. The data thus indicate that the multiplicity of TSH binding proteins demonstrated in many labs may be breakdown products of a receptor which is synthesized by a single message but has both 330 and 70 K forms and is tightly complexed with a specific thyroid ganglioside. The 70 K form is composed of ~ 50 and ~ 20 K fragments seen in TSH cross-linking studies.