The glycine receptor of rat spinal cord is an oligomeric membrane glycoprotein of molecular mass 250,000 daltons that contains three polypeptides of 48,000, 58,000, and 93,000 daltons. Monoclonal antibodies (mAbs) were prepared against the affinity-purified glycine receptor protein by using 125I-labeled receptor preparations for the detection of positive hybrids. From nine monoclonal antibodies obtained, six recognized denatured receptor polypeptides blotted to nitrocellulose paper. Two of these antibodies bound to more than one glycine receptor polypeptide: mAb GlyR 4a stained the 48,000- and 58,000-dalton polypeptides, and mAb GlyR 7a stained the 48,000- and 93,000-dalton polypeptides. Common antigenic determinants thus are shared by the different subunits of the glycine receptor. Complementary results were obtained by peptide mapping of 125I-labeled glycine receptor polypeptides with various proteases. A set of peptide fragments of the same apparent molecular mass was produced from the different glycine receptor polypeptides by using V8 protease, chymotrypsin, and elastase. These data suggest that the subunits of the glycine receptor have significant homology within their primary structure and may have evolved from a common ancestor receptor polypeptide.