Exocyclic peptide mimetics that disable Fas were developed. A three dimensional model of the Fas receptor-ligand complex was constructed and structurally predicted regions of the receptor that were relevant to binding ligand were used to create constrained peptide mimetics. Exocyclic anti-Fas peptide mimetics were identified that block Fas receptor-ligand interactions, and modulate Fas biological activity both in vitro and in vivo. The mimetics are useful, e.g., for treating Fas-related pathologies.