Mast cells and eosinophils are closely associated in allergic reactions. Eosinophil peroxidase (EPO) when combined with hydrogen peroxide (H202) and a halide forms a strong oxidative system capable of initiating mast cell degranulation. EPO also binds firmly to the secreted mast cell granules (MCG) to form a complex with augmented peroxidatic activity. Objectives of this research are 1) to study the formation and biological properties of the MCG-EPO complex; 2) to study the formation and inactivation of leukotrienes by phagocytes; 3) to study the induction of mast cell secretion by agents released by granulocytes; 4) to study the interaction of mast cell and eosinophil products in the killing of the schistosomula of Schistosoma mansoni and 5) to study the degradation of mast cell heparin by granulocyte products. The overall aim is to study the interaction of mast cells and eosinophils and the role of this interaction in the manifestations of allergic diseases and in the host defense against parasites.