In studies of disease processes, increasing knowledge leads to an increased awareness of the complexity of the underlying mechanisms. The intense research activity in the chemokine field has made this acutely manifest. Numerous chemokines have been discovered through the use of (1) bioassay of in vitro cell culture supernatants and in vivo exudates from animal models of inflammation and (2) molecular biology techniques. Any one chemokine can often be produced by a number of different cell types and exert its effects on different target cells. This has been interpreted by some as implying a high degree of redundancy. Although this is understandable, in disease processes parallel and sequential mechanisms are possible, and potentially important therapeutic targets have emerged. There is compelling evidence from animal and clinical studies that eosinophils are important effector cells in asthma, but this relationship is as yet unproven in the human disease. Two possible targets to prevent eosinophil recruitment to the lung are IL-5 and its receptor, which are important in several aspects of eosinophil biology, and eotaxin and its receptor, CCR3. The eotaxin receptor is particularly attractive as a target as it is expressed in high numbers on eosinophils, but not other leukocytes, and appears to be the major detector of the eosinophil for eotaxin and other chemokines such as MCP-4. Eotaxin and CCR3 knockout mice are being developed, and animal models will continue to be invaluable when antagonists are available. In the shape of receptor antagonists, the chemokine field may yet provide the final proof of concept for the long-established eosinophil theory of asthma in humans.