Peptides can be developed as effective antagonists of protein-protein interactions, but conventional peptides (i.e., oligomers of L-alpha-amino acids) suffer from significant limitations in vivo. Short half-lives due to rapid proteolytic degradation and an inability to cross cell membranes often preclude biological applications of peptides. Oligomers that contain both alpha- and beta-amino acid residues ("alpha/beta-peptides") manifest decreased susceptibility to proteolytic degradation, and when properly designed these unnatural oligomers can mimic the protein-recognition properties of analogous "alpha-peptides". This report documents an extension of the alpha/beta-peptide approach to target intracellular protein-protein interactions. Specifically, we have generated alpha/beta-peptides based on a "stapled" Bim BH3 alpha-peptide, which contains a hydrocarbon cross-link to enhance alpha-helix stability. We show that a stapled alpha/beta-peptide can structurally and functionally mimic the parent stapled alpha-peptide in its ability to enter certain types of cells and block protein protein interactions associated with apoptotic signaling. However, the alpha/beta-peptide is nearly 100-fold more resistant to proteolysis than is the parent stapled alpha-peptide. These results show that backbone modification, a strategy that has received relatively little attention in terms of peptide engineering for biomedical applications, can be combined with more commonly deployed peripheral modifications such as side chain cross-linking to produce synergistic benefits.