Considerable progress has been achieved on mapping the receptor binding site of cationic biogenic amine hormones, however, little progress has been made on determining the nature of the binding site of G-protein coupled receptors that bind peptide ligands of sizes that range from three (fMLP) to several hundred residues (LH and FSH). It is clear that all G- protein coupled receptors share some common structural features, however, there are also major differences with regard to the binding domain. In this proposal we choose to elucidate the binding domain of the interleukin-8 receptor which belongs to the superfamily of G-protein coupled receptors. Interleukin-8 (IL-8) is a 72 amino acid peptide secreted by many cell types in response to proinflammatory stimuli such as interleukin-1, and tumor necrosis factor. IL-8 induces transendothelial migration cloning of Interleukin-8 receptor subtypes exhibiting novel ligand binding specificity with regard to structurally related peptides, neutrophil activating peptide 2 (NAP-2) and melanoma growth stimulating activity (MGSA). We determined that the ligand binding specificity is dictated by the extracellular N-terminus to identify the residues at the IL-8/receptor interface by using a genetic approach. Mapping the amino acid residues at the ligand-receptor interface will aid toward the design of novel IL-8 receptor antagonists that will impact in the treatment of inflammatory disorders.