C5a is a 74 amino acid polypeptide that likely plays an important role in the pathogenesis of a number of inflammatory diseases. Therefore, the discovery of a C5a antagonist is of considerable therapeutic interest. A series of peptides designed to survey various regions of the molecule was synthesized by solid-phase peptide synthesis and evaluated for receptor-binding activity with polymorphonuclear leukocyte membranes. The C-terminal octapeptide (Ac-His-Lys-Asp-Met-Gln-Leu-Gly-Arg-OH) was identified as the smallest fragment which possessed reasonable C5a receptor binding activity.