Monocyte Chemotactic Protein (MCP-3) is a novel chemokine that has been recently purified from human osteosarcoma cell line1. It was shown that MCP-3 is produced by both tumours cells1 and leukocytes.2,3 MCP-3 can bind and activate a vast diversity of inflammatory cell types through interaction with multiple leukocyte receptors as well as it’s own receptor.6,7 Murine MCP-3 Marc/Fic chemokines that have been previously cloned are thought to be homologues of human MCP-3.3,4 MCP-3 cDNA cloning2 and structural analysis revealed that this 76 amino acid (a.a.) polypeptide with a molar mass of 8.5 kDa belongs to a family of small inflammatory proteins, characterized by four conserved cysteine residues and is localized on human chromosome 17.1,4,5 MCP-3 is designated a C-C or intercrine cytokine.13 MCP-3 can indeed use a wide variety of binding sites and activate many inflammatory cells. In vitro, both MCP-3 and MCP-1 can activate T-cells, monocytes, and basophils, but only the first can activate eosinophils. These cytokines show 71% a.a. homology. MCP-3 was also revealed to have 30% a.a homology with MIP-1 and RANTES which can also activate eosinophils plus all cell types mentioned above.6,7 MCP-3 seems to be the only C-C chemokine that regularly induces neutrophil migration. It is also a potent chemoattractant for human dendritic cells.7 It has been suggested that MCP-3 binds multiple C-C receptors such as MCP-1 on monocytes and basophils 8,9, MIP-1a on neutrophils, basophils, and eosinophils9 and RANTES on basophils and eosinophils(9,10). Evidence suggests that MCP-3 does indeed use multiple receptors (MCP-1, RANTES, and MIP-1a) and binds with MIP-1 receptor as well as other unique binding sites 6,7.MCP-3 was discovered to be the strongest C-C chemokine in inducing the migration of C-C CKR1 transfected cells. It was shown that MCP-3 binds with C-C CKR1 receptor with greater affinity than MIP-1a or Rantes, which mainly activate it 6,11. Also, MCP-3 promotes exocytosis of eosinophil granule proteins and stimulates histamine release from human basophils 4,12. In vivo, MCP-3 induces the selective infiltration of monocytes on intradermal injection in rabbits.1 Since MCP-3 acts on a variety of inflammatory cells and utilizes multiple receptors for its function, characterization and isolation of the shared as well as unique receptors for MCP-3 will provide further insights into the pathophysiological roles of MCP-3. C-C chemokines are mediators of a number of pathological conditions such as chronic inflammation, tumor, allergy, as well as atherosclerosis10. Since the binding and signaling of MCP-3 is most promiscuous, the development of antibody or antagonist which can block MCP-3 through binding to the receptor or ligand may prove to be useful in the treatment of diseases mediated by a number of C-C chemokine. This MCP-3 ELISA is a 2.5 hour solid phase immunoassay readily applicable to measure MCP-3 in serum, plasma, cell culture supernatant, and other biological fluids in the range of 0 to 1600 pg/mL. It showed no cross-reactivity with other cytokines tested. MCP-3 may play a role in certain diseases, therefore this MCP-3 ELISA is expected to be effectively used for further investigations into the relationship between MCP-3 and various pathological conditions.