Platelet-activating factor (PAF) is one of the chemical mediators that may participate in the inflammatory process underlying asthma. It has been shown to produce a prolonged increase in airway responsiveness in several species, including humans. In humans the increase in bronchial reactivity peaks 3 days after inhalation and can last as long as several weeks. PAF may induce this response by recruiting eosinophils into the airways and activating them in the airway walls, with the subsequent release of basic proteins. This process causes eosinophils, which are rich sources of PAF, to release their contents and sets up a self-perpetuating cycle. Nedocromil sodium reduces the increased bronchial responsiveness of asthma and can prevent the development of increased responsiveness after inhaled PAF. The precise mechanism by which this result occurs is not known and requires further investigation.