Purification techniques were developed for mouse eosinophils and neutrophils. Membrane receptors on mouse eosinophils and neutrophils were studied by means of rosette formation, phagocytosis and and 51 Cr release assays using mouse complement and monoclonal IgM, IgGM, and IgG2b antibody-coated sheep erythrocytes. Eosinophils as well as neutrophils were found to possess complement and IgG receptors, with eosinophils showing higher complement and antibody requirements than neutrophils. Both cell types reacted more strongly with IgG2b than with the IgGl used, but it is unclear whether this is a subclass effect or a reflection of a different antibody density on the sheep red cell membrane. The possibility that lysis of trypanosomes in the acute phase of Trypanosoma cruzi infection may result in T.cruzi antigen coating of host cells, thus rendering them susceptible to the host effector mechanisms, was investigated. An in vitro model was used, in which mouse eosinophils and neutrophils were found to be cytotoxic against mouse cell lines coated with T.cruzi antigen in the presence of anti-T.cruzi antibody. The finding that eosinophils could kill mammalian cells led to experiments in which rat eosinophils, neutrophils and K cells were tested against antibody-coated mouse cell line cells. All three effector cells were found to be active against cells of lymphoid origin. Cytotoxicity by granulocytes was shown to be specific for the antibody-coated target cells and to depend on the type and concentration of the antibody preparation used. The production of a monospecific anti-eosinophil serum that would allow studies on the role of the eosinophil in experimental protozoan infections was attempted by hyperimmunizing rabbits with highly purified preparations of mouse eosinophils. The antisera thus obtained were found to cross-react with other leucocytes and, even after absorption with a range of mouse cells, no specificity for eosinophils was achieved. Monoclonal antibody techniques were then tried, and rat-mouse and rat-rat fusions produced several hybridomas secreting anti-eosinophil antibodies. One of these has been shown to be highly specific in vitro and to selectively deplete eosinophils in vivo.