The isolated subunits of the acetylocholine receptor from Torpedo californica were digested with proteolytic enzymes, and the resulting polypeptide fragments were analyzed by gel electrophoresis. We have identified those fragments which contain carbohydrate and those from the alpha subunit which are labelled with the acetylcholine binding site specific reagent [4-(N-maleimido)benzyl]tri[3H]methylammonium iodide. We have tested several monoclonal antibodies raised to the acetylcholine receptor from torpedo, some of which react with the denatured subunits [Tzartos, S.J., & Lindstrom, J.M. (1980) Proc. Natl. Acad. Sci. U.S.A.77, 755; Tzartos, S.J., & Lindstrom, J.M. (1981) in Monoclonal antibodies in Endocrine Research (Fellows, R., & Eisenbarth, G., Eds.) Raven Press (in press)]. The binding specificities of these antibodies to radioiodinated proteolytically generated fragments of the alpha subunit were determined by immunoprecipitation followed by gel electrophoresis. The antibodies tested fell into at least three main groups on the basis of their binding specificities. These antibodies were also tested for their capacity to bind to acetylcholine receptor solubilized in Triton X-100, sodium cholate, or sodium cholate supplemented with exogenous lipids. A monoclonal antibody raised to the denatured delta subunit, was tested for its ability to select radioiodinated proteolytic fragments of these subunits. These molecules provide probes for many sites on the acetylcholine receptor with affinities and specificities comparable to alpha-neurotoxins.