After analysis of the technical parameters of the rosette assay with human IgE-coated erythrocytes, Fc epsilon receptors for IgE (Fc epsilon R) on human peripheral blood eosinophils were compared to Fc epsilon R on lymphocytes and monocytes. Antibodies directed against Fc epsilon R on lymphocytes and monocytes inhibited the IgE rosettes formed by eosinophils from hypereosinophilic patients, which suggests that Fc epsilon R on eosinophils were antigenically related to Fc epsilon R on lymphocytes and monocytes. Fc epsilon R on human eosinophils were shown to participate in the killing effect of Schistosoma mansoni schistosomula in vitro in the presence of purified eosinophils from highly hypereosinophilic patients (blood counts greater than 3000/mm3) and anti-schistosomula IgE antibodies present in S. mansoni-infected patient sera. Similar levels of inhibition of cytotoxicity were obtained after preincubation of eosinophils with aggregated human IgE or with anti-Fc epsilon R antibodies, whereas preincubation with aggregated IgG or with anti-C3b receptor antibodies did not decrease the killing effect for schistosomula targets. This IgE-dependent cytotoxic capacity seemed restricted to eosinophils with an abnormally low density ("hypodense" cells) present only in highly hypereosinophilic patients. These observations might be related to nonparasitic situations in which increased levels of IgE and tissue or blood eosinophils are observed.