The objective of the research is to characterize the eosinophil peroxidase system which consists of the granular enzyme eosinophil peroxidase, the cellular product hydrogen peroxide, and an oxidizable co-factor, supplied as iodide. An active peroxidase system has been identified in human and guinea pig peritoneal exudate eosinophils. Methods for measurement of hydrogen peroxide production by eosinophils have been developed. Anaerobic studies have shown that the peroxidase system does not participate in eosinophil - mediated damage of schistosomula. Further studies will explore hydrogen peroxide production, as well as functions of the peroxidase system.