The N terminus of G protein-coupled receptors has been implicated in binding to peptide hormones. We have used random saturation mutagenesis to identify essential residues in the N terminus of the human complement factor 5a receptor (C5aR). In a library of N-terminal mutant C5aR molecules screened for activation by C5a, residues 24-30 of the C5aR showed a marked propensity to mutate to cysteine, most likely indicating that sulfhydryl groups at these positions are appropriately situated to form disulfide interactions with the unpaired Cys27 of human C5a. This presumptive spatial constraint allowed the ligand to be computationally docked to the receptor to form a model of the C5a/C5aR interaction. When the N-terminal mutant C5aR library was rescreened with C5a C27R, a ligand incapable of disulfide interactions, no individual position in the N terminus was essential for receptor signaling. However, the region 19-29 was relatively highly conserved in the functional mutants, further demonstrating that this region of the C5aR makes a productive physiologic interaction with the C5a ligand.