We have measured airways reactivity (methacholine PC20) and blood eosinophils in eleven asthmatics with allergen-induced late-phase reactions. Before challenge there was a significant (P less than 0.05) inverse correlation between blood eosinophils and the methacholine PC20. After allergen inhalation eosinophil counts were significantly (P less than 0.01) elevated at 24 hr. These increases in blood eosinophil counts were not observed in patients who developed single early responses. The magnitude of the changes in eosinophil counts (24 hr minus pre-challenge values) also correlated with the baseline methacholine PC20 (P less than 0.01) in the late-phase responders. These observations suggest that there may be a direct association between eosinophils and airway reactivity in subjects who develop late-phase asthmatic reactions.