The activity of eosinophils and neutrophils with respect to the release of granule proteins was studied in 11 patients with the hypereosinophilic syndrome (HES). Granulocytes or purified eosinophils were stimulated with serumopsonized Sephadex particles (C3b-induced release), and the released amounts of eosinophil cationic protein (ECP), eosinophils protein-X (EPX) and myeloperoxidase (MPO) were measured by means of specific radioimmunoassays (RIA). Eosinophils obtained from patients with HES released significantly more ECP (P < 0.002) and EPX (P < 0.01) after 20 min of incubation than cells from the control group. The cellular content of ECP and EPX in eosinophils obtained from the patients with HES was significantly reduced to 50% and 62%, respectively, of the content of these granule proteins of eosinophils from the control group. In separated eosinophils light-density eosinophils released more of both ECP and EPX than normal density eosinophils. There was no difference in MPO release between the patients and the control group. We conclude that the eosinophils from patients with HES have an increased propensity to release their granule proteins and the releasability seems to be related to the density of the cells.