Rat eosinophils or neutrophils were purified from peritoneal washings which had been enriched either for eosinophils by infection with the parasite Mesocestoides corti or by intravenous injection with Sephadex G200 particles, or for neutrophils by the intraperitoneal injection of glycogen. Neither eosinophils nor neutrophils attached to or damaged live M. corti parasites in vitro although they did lyse chick erythrocytes in the presence of rat anti-chick red blood cell antibody, with the neutrophils showing the highest level of cytotoxicity and the eosinophils from the infected rats the lowest. Neutrophils gave a specific antibody-dependent cytotoxic response to chick erythrocytes coated with solubilized M. corti extract, a response not seen with eosinophils. The cytotoxicity shown by neutrophils could not be blocked by adding eosinophils or sera obtained from chronically infected rats although it was reduced by incubating the neutrophils with cell-free supernatants obtained from the spleen cells of infected rats following stimulation with solubilized parasite extract in vitro. Eosinophils from infected rats expressed fewer membrane Fc receptors for antibody than did neutrophils or eosinophils from uninfected animals. Incubation of neutrophils and eosinophils from uninfected rats with the immune spleen cell supernatants reduced Fc receptor expression to levels similar to those seen with eosinophils from infected animals. These same supernatants had no effect on the expression of granulocyte complement receptors. It is suggested that infection of rats with M. corti can lead to the production of an antigen-specific suppression capable of impairing the antibody-dependent activity of granulocytes in vitro.