AbstractWe have determined which amino acids contribute to the pharmacophore of human C5a, a potent inflammatory mediator. A systematic mutational analysis of this 74‐amino acid protein was performed and the effects on the potency of receptor binding and of C5a‐induced intracellular calcium ion mobilization were measured. This analysis included the construction of hybrids between C5a and the homologous but unreactive C3a protein and site‐directed mutagenesis. Ten noncontiguous amino acids from the structurally well‐defined 4‐helix core domain (amino acids 1–63) and the C‐terminal arginine‐containing tripeptide were found to contribute to the pharmacophore of human C5a. The 10 mostly charged amino acids from the core domain generally made small incremental contributions toward binding affinity, some of which were independent. Substitutions of the C‐terminal amino acid Arg 74 produced the largest single effect. We also found the connection between these 2 important regions to be unconstrained.