The presence of cytoplasmic lipid bodies in human eosinophils and the participation of these lipid bodies in the metabolism of arachidonic acid by human eosinophils have been studied. Lipid bodies, structures of roughly spherical shape and variable size within the cytoplasm, were identified with transmission electron microscopy by their shape and variable osmiophilia and by their lack of a limiting membrane. While generally absent from eosinophils of normal peripheral blood, lipid bodies were found in tissue eosinophils and in blood eosinophils from patients with eosinophilia. A role for lipid bodies in arachidonic acid metabolism was determined with eosinophils obtained from two eosinophilic patients. After incubation for 30 to 60 minutes with 3H- arachidonic acid, purified eosinophils took up 50% to 98% of the tritium label. By electron microscopic autoradiography, almost all tritium label was localized to lipid bodies. Only 3.6% of the cell- incorporated tritium label was free arachidonic acid, while 5.8% was neutral lipids and 66% was phospholipid. Thus, most of the tritiated arachidonic acid incorporated by human eosinophils was present in esterified form, predominantly as phospholipids, and almost all of the tritiated lipids were localized ultrastructurally to cytoplasmic lipid bodies. These results provide evidence that lipid bodies of human eosinophils have a role in the cellular metabolism of arachidonic acid.