The study was carried out to identify those molecules that are important in vivo in the attraction of eosinophil granulocytes to the lungs of patients with asthma. Asthmatic patients with birch pollen allergy had lavages performed before and during the pollen season, and the chemotactic activity of the bronchoalveolar lavage fluid was tested against normal eosinophils. The activity was significantly increased during the pollen season as compared with the activity before the pollen season (p less than 0.01). Neutralizing antibodies to IL-2, IL-5 and IL-8, leukemia inhibitory factor, and to RANTES were added to the bronchoalveolar lavage fluid. Antibodies to IL-5 and RANTES, but not to IL-2 and IL-8 or leukemia inhibitory factor, significantly inhibited the chemotactic activity for eosinophils (p less than 0.001). It is concluded that IL-5 and RANTES are important chemoattractants in the lungs of patients with allergic asthma. The effect of IL-5 may be that of a cofactor to the chemotactic molecules, of which RANTES may be one of the most important in allergic asthma.