The molecular cloning of the C5a receptor places this molecule in the superfamily of G-protein coupled receptors. This superfamily is characterized by the presence of signature motifs including seven hydrophobic domains which span the cell membrane and impart a predicted serpentine topology to the receptor proteins. The identification of other members of this family, including receptors for the chemokines IL-8 and Mip-1/Rantes, thrombin, formyl peptide, and platelet activating factor, provide new tools for understanding structure-function relationships relevant to the inflammatory process. This review focuses on the recent biological studies concerning the ligand C5a and its cellular receptor, the structure/activity relationships so far discerned, signal transduction mechanisms, progress toward identification of receptor antagonists, and some likely directions for future studies. Where appropriate, relevant work on related seven transmembrane segment receptors is discussed.