Eosinophils from a patient with hypereosinophilia were observed to phagocytize radiolabeled E. coli or S. aureus as effectively as neutrophils from a normal control. This was observed at a number of bacteria/cell ratios and a various time intervals following initial challenge and was confirmed by direct microscopic examination. In spite of comparables rates of phagocytosis, the eosinophils were consistently less capable of killing the bacteria. This correlates with an inability of eosinophil peroxidase to catalyze the peroxidase-H2O2-CI--mediated decarboxylation of amino acids; in contrast, both eosinophil and neutrophil peroxidases showed similar capabilities to iodinate protein in vitro. These data are compatible with the importance of the chloride- mediated mechanism in the bactericidal activity of intact phagocytes.