The effect of pretreatment with rat anti-murine interleukin-5 (IL-5) antibody on antigen-induced bronchial eosinophilia and bronchial reactivity to acetylcholine in mice were studied. Three inhalations of an antigen by actively sensitized animals resulted in an increase in airway reactivity to acetylcholine. Twenty-four hours after the final inhalation, the number of leukocytes (mononuclear cells and eosinophils) and the amount of IL-5 in BALF increased significantly. Anti-IL-5 monoclonal antibody inhibited the antigen-induced increase of eosinophils with little effect on bronchial hyperreactivity.