Asthma is a common and complex inflammatory disease of the airways that remains incurable. Current forms of therapy are long term and may exhibit associated side-effect problems. Major participants in the development of an asthma phenotype include the triggering stimuli such as the allergens themselves, cells such as T cells, epithelial cells and mast cells that produce a variety of cytokines including IL-5, GM-CSF, IL-3, IL-4 and IL-13 and chemokines such as eotaxin. Significantly, the eosinophil, a specialized blood cell type, is invariably associated with this disease. The eosinophil has long been incriminated in the pathology of asthma due to its ability to release preformed and unique toxic substances as well as newly formed pro-inflammatory mediators. The regulation of eosinophil production and function is carried out by soluble peptides or factors. Of these IL-5, GM-CSF and IL-3 are of paramount importance as they control eosinophil functional activity and are the only known eosinophilopoietic factors. In addition they regulate the eosinophil life span by inhibiting apoptosis. While one therapeutic approach in asthma is directed at inhibiting single eosinophil products such as leukotrienes or single eosinophil regulators such as IL-5, we believe that the simultaneous inhibition of more than one component is preferable. This may be particularly important with eosinophil regulators in that not only IL-5, but also GM-CSF has been repeatedly implicated in clinical studies of asthma. The fact that GM-CSF is produced by many cells in the body and in copious amounts by lung epithelial cells highlights this need further. Our approach takes advantage of the fact that the IL-5 and GM-CSF receptors (as well as IL-3 receptors) utilize a shared subunit to bind, with high affinity, to these cytokines and the same common subunit mediates signal transduction culminating in all the biological activities mentioned. By generating the monoclonal antibody BION-1 to the cytokine binding region of the common subunit (betac) we have shown that the approach of inhibiting IL-5, GM-CSF and IL-3 binding and the resulting stimulation of eosinophil production and function with a single agent is feasible. Furthermore we have used BION-1 as a tool to crystallize and define the structure of the cytokine binding domain of betac. This knowledge and this approach may lead to the generation of novel therapeutics for the treatment of asthma.