Identification of short, structured peptides able to mimic potently protein-protein interfaces remains a challenge in drug discovery. We report here the use of a naive cyclic peptide phage display library to identify peptide ligands able to recognize and mimic IgG1-Fc functions with Fc gammaRI. Selection by competing off binders to Fc gammaRI with IgG1 allowed the isolation of a family of peptides sharing the common consensus sequence TX(2)CXXthetaPXLLGCPhiXE (theta represents a hydrophobic residue, Phi is usually an acidic residue, and X is any residue) and able to inhibit IgG1 binding to Fc gammaRI. In soluble form, these peptides antagonize superoxide generation mediated by IgG1. In complexed form, they trigger phagocytosis and a superoxide burst. Unlike IgG, these peptides are strictly Fc gammaRI-specific among the Fc gammaRs. Molecular modeling studies suggest that these peptides can adopt 2 distinct and complementary conformers, each able to mimic the discontinuous interface contacts constituted by the Cgamma2-A and -B chains of Fc for Fc gammaRI. In addition, by covalent homodimerization, we engineered a synthetic bivalent 37-mer peptide that retains the ability to trigger effector functions. We demonstrate here that it is feasible to maintain IgG-Fc function within a small structured peptide. These peptides represent a new format for modulation of effector functions.