Eosinophils participate in the inflammatory response seen in allergy and helminthic infestation. Their release of granule-bound cationic proteins may play a role in these diseases. Therefore, we investigated mechanisms involved in the release of eosinophil cationic protein (ECP). Serum-opsonized zymosan was phagocytosed by eosinophils, and ECP was released into the phagosomes as judged by immunoelectron microscopy. Degranulation to the external milieu was induced by serum-opsonized, non-phagocytosable Sephadex beads (SOS), and ECP release was determined by use of an enzyme-linked immunosorbent assay. CD11b, CD18, and CD32 monoclonal antibodies inhibited degranulation, demonstrating dependence on complement receptor type 3 (CR3), and the low-affinity Fc receptor for IgG. Tumor necrosis factor-alpha (TNF-alpha) and interleukin (IL)-5 both rapidly enhanced the binding of eosinophils to serum-opsonized zymosan, and also the release of ECP upon interaction with SOS. The cytokine-induced increase in ECP release was inhibited by the phospholipase A2 (PLA2) inhibitor mepacrine, indicating an involvement of PLA2 in the enhanced response but not in baseline degranulation. Autocrine stimulation by the platelet-activating factor (PAF) is unlikely since the PAF receptor antagonist WEB 2086 did not inhibit the enhanced response. In conclusion, the main signals for eosinophil degranulation on serum-opsonized particles are mediated by CR3 and receptors for immunoglobulins. As for IL-5, TNF-alpha changes eosinophil phenotype from a resting to an activated state.