In the last 10 years it has become generally accepted that asthma is an inflammatory disease of the airways. T lymphocytes play a key role in regulating this inflammatory response, through the elaboration of cytokines such as IL-4, IL-5, and IFN-gamma. IL-5 is of particular interest because it is the major eosinophil survival factor in vivo. Eosinophils are felt to be the major effector cells in asthma, causing damage to various airway structures.