An extensive structure-activity study of synthetic analogues of the C3a anaphylatoxin was conducted. Our goal was to map C3a-C3a receptor interactions by designing synthetic analogue molecules having maximal biologic potency. Nonspecific binding of the polycationic C3a to polyanionic molecules on cellular surfaces often obscures specific binding to the receptor. Less cationic synthetic C3a analogues would be useful tools in identifying and characterizing the various cell types having C3a receptors. These factors should also be useful as pharmacologic probes for mechanism studies, as high-affinity ligands for target cell identification, and for receptor isolation. Attachment of amino-terminal hydrophobic groups such as Fmoc to C3a analogues [as orginally introduced by Gerardy-Schahn et al. (1988) Biochem. J. 255, 209] markedly enhanced the potency of synthetic C3a peptides. The enhancement effect on potency from introducing hydrophobic groups to C3a analogues was interpreted as possibly being nonspecific. Our systematic search for an optimal peptide length, composition, and N-terminal hydrophobic unit resulted in several superpotent C3a analogues having 200-1500% the potency of natural C3a. One particularly potent C3a peptide was designed by incorporating two tryptophanyl residues at the N-terminal end of a 15-residue C3a analogue. The superpotent peptide W-W-G-K-K-Y-R-A-S-K-L-G-L-A-R has several residues differing (underlined) from the sequence corresponding to positions 63-77 in human C3a, a region that contains the essential functional site of the molecule. This 15-residue model peptide exhibited the greatest biological potency of all peptides tested, being 12-15 times more active than natural C3a. Since an optimal distance was found to exist between the N-terminal hydrophobic unit (W-W) and the C-terminal primary binding site (LGLAR), we concluded that the hydrophobic unit interacts specifically with a secondary binding site on the C3a receptor. The presence of both a primary (effector) and secondary (hydrophobic) binding site on these linear synthetic ligands, which can interact cooperatively with the C3a receptor, presumably accounts for the high relative potency of the analogues. Our design of superpotent analogues of C3a demonstrates the feasibility for constructing small synthetic peptides to mimic natural biologic factors that depend on secondary or tertiary structure for their activity. These synthetic peptide studies demonstrate that a linear array of amino acids (e.g., W-W) can successfully substitute for a conformation-dependent binding site on a bioactive factor.