To characterize the importance of a highly conserved region of the class II β chain, we introduced an amino acid substitution that is predicted to eliminate a hydrogen bond formed between the class II molecule and peptide. We expressed the mutated β chain with a wild-type α chain in a murine L cell by gene transfection. The mutant class II molecule (81βH⁻) assembles normally in the endoplasmic reticulum and transits the Golgi complex. When invariant chain (Ii) is coexpressed with 81βH⁻, the class II–Ii complex is degraded in the endosomes. Expression of 81βH⁻ in the absence of Ii results in a cell surface expressed molecule that is susceptible to proteolysis, a condition reversed by incubation with a peptide known to associate with 81βH⁻. We propose that 81βH⁻ is protease sensitive because it is unable to productively associate with most peptides, including classII–associated invariant chain peptides. This model is supported by our data demonstrating protease sensitivity of peptide-free wild-type I-A^d molecules. Collectively, our results suggest both that the hydrogen bonds formed between the class II molecule and peptide are important for the integrity and stability of the complex, and that empty class II molecules are protease sensitive and degraded in endosomes. One function of DM may be to insure continuous groove occupancy of the class II molecule.