Cytokines, such as interleukin-3 (IL-3), have been suggested to play an important role in mediating the increased number of airway eosinophils and metachromatic cells in patients with even mild asthma. We used immunohistochemistry to determine the presence of IL-3 protein in bronchial biopsies from nonasthmatics (n = 10) and subjects with mild (n = 8) and allergen-induced (n = 7) asthma. We also examined whether IL-3 was related to airway eosinophil number and activation, the number of airway metachromatic cells, or airway function. We found that the number and activation of eosinophils and the number of metachromatic cells were increased in the airways of asthmatics, compared with nonasthmatics, with further increases evident after allergen challenge. IL-3 protein was localized primarily to the epithelium in nonasthmatic and asthmatic subjects, with no difference apparent between groups or after allergen inhalation challenge. The extent of staining for IL-3 in the tissue was not correlated with eosinophil number or activity, metachromatic cell number, airway responsiveness, or the severity of the late asthmatic response. This study provides the first demonstration of IL-3 protein localization in bronchial tissue from human airways. The results suggest that the increases in eosinophils and metachromatic cells associated with mild and allergen-induced asthma occur independent of IL-3.