Recent work has suggested that there may be heterogeneity amongst human eosinophils. In order to study this further, blood and tissue eosinophils were separated by density gradient centrifugation in discontinuous gradients of metrizamide. Blood eosinophils were obtained from 16 patients with blood eosinophil counts above 1 X 10(9)/l. Resident eosinophils were isolated from alveolar fluid in three of these patients and from pleural effusion in two others. A large proportion of the eosinophils in these patients were of light buoyant density (hypodense) and a high percentage of these showed morphological evidence of hypogranulation. Hypodense eosinophils had a reduced capacity to reduce nitroblue tetrazolium or develop chemiluminescence after stimulation with phorbol myristate acetate. It was concluded that there is structural and metabolic heterogeneity amongst blood and tissue eosinophils in patients with a variety of eosinophilic disorders.