To study the role of IL-5 in allergic airway hyperreactivity, the time course for the production of cytokines, the infiltration of inflammatory cells and the onset of airway hyperreactivity after three inhalations of antigens were studied in mice. The effect of the soluble alpha-chain of murine recombinant interleukin-5 receptor (sIL-5R alpha) on these phenomena was also examined. Whereas IL-5 and IL-4 were produced in significant amounts, IL-1, IL-2 and gamma-interferon (gamma-IFN) were not detected even after three antigen inhalations. Monocytes and eosinophils but not neutrophils increased significantly after the third antigen exposure. The airway responsiveness to acetylcholine increased after the third aeroantigen-challenge. sIL-5R alpha, administered after each antigen-challenge, suppressed BAL eosinophilia with little effect on airway hyperreactivity.